Installing Ubuntu to a removable USB drive with Virtual Box

In order to install Ubuntu to a portable external USB drive (either disk or stick) we may also use Virtual Box to install from a virtual environment. For USB 2.0 support the closed source but free PUEL-version of Virtual Box is needed.

Create a virtual machine for the installation live environment:

We create a virtual machine for a Linux/Ubuntu environment (32- or 64-bit, depending on the installation medium):

As we want to install to an USB drive we do not create a virtual harddisk (VDI) for this machine by unticking the box in the following window:

We then need to assign system memory (e.g. 1024 MB), graphics memory (e.g. 128MB), and adjust CPU settings according to our host hardware. Also we may want to create a bridged network in order to be able to download files during the installation.

Mount the installation CD to the virtual machine:

In the Storage menu from Virtual Box Manager we select the .iso image of our installation CD to mount as CD drive. Make sure the boot order of the virtual machine is set to boot from CD.

Mount the USB drive to the installation environment

After we started the virtual machine (USB support needs to have been set up first) to boot the installation CD we need to mount the USB drive either by clicking on the small icon in the bottom panel or by choosing from Devices -> USB Devices menu of Virtual Box Manager.

^{This is when the USB drive needs to have been mounted before we proceed}

Partition and format the USB drive

After having chosen Something else the graphical partition manager GParted will guide us through the partitioning process:

We need at least a partition with a mount point root (/). In the example above an additional /home partition was created. By unticking Format we keep the data that may already be there. A /swap partition may not be needed for an USB-stick or a portable drive.

• At this point take extra care that the boot loader Grub indeed will be installed to the USB drive (/sda) and not to anywhere else

By selecting Install Now we start the installation to our USB drive. Consider that this installation may take a bit longer than we are used to.

After the installation has finished we may unmount our drive eith the brand new operating system and boot from any other machine to customized it to our needs.

• 
  Do not forget to enable booting from USB in this computer's BIOS.

If you are talking about an actual install, as in a full Ubuntu install rather than just a Live USB type then what you can do is use an external hard drive that plugs in via USB and install to that via the following method.

Please Note: The following steps were tested using Ubuntu Version 9.10, but has not been tested with the later versions. Use at your own risk & discretion.

What You Will Need

1. A Computer with Internet access.


2. A LiveCD or LiveUSB with Ubuntu.


3. An external Hard Drive with USB capability.

What To Do

1. Open up your computer and remove the Hard Drive.


2. Plug in your external USB Hard Drive via the USB cable.


3. Stick in your LiveUSB or LiveCD and then boot up your PC.


4. Open up the boot menu, and choose to boot from the LiveCD/LiveUSB.


5. During the installation process you should your external hard drive listed, install Ubuntu to that.


6. Finish the installation process, turn off your PC, and put your other hard drive back into your computer.


7. Reboot your computer, go to the boot menu and select your external hard drive and attempt to boot from it. If it does congratulations, you now have an external hard drive with a full fledged Operating System on it.


8. Enjoy your external hard drive running Ubuntu/Linux!
   Please do let me know if this helps you! If not let me know about that too. :)

But if you're just wanting a Live USB then you can use the Universal USB Installer for that or the Ubuntu USB Startup Disk Creator...

Introduction

The main part of this step-wise instruction is borrowed from the iso testing tracker and this link,

• Install (entire disk) in Ubuntu Desktop amd64 in Xenial 16.04.1

and I have added some extra steps necessary for the installation to an external drive.

• 
  

  Please notice that you will install a system, that works in the current boot mode,

  
  
  
  
  
  • If you install in UEFI mode, the installed system will work in UEFI mode
  
  
  • If you install in BIOS alias CSM alias legacy mode, the installed system will work in BIOS mode.
  
  
  
  


• 
  

  It is more complicated to create an installed system, that will work both in UEFI and BIOS mode, but it is possible according to the following link and links from it,

  
  
  

  A portable installed system, that boots both in UEFI and BIOS mode

  
  

• 
  

  If you intend to install into a USB pendrive or memory card, choose a fast USB 3 pendrive or a high-speed card. See this link and links from it,

  
  
  

  help.ubuntu.com/community/Installation/FromUSBStick#Notes_about_speed

  
  

• If you intend to use the external drive in new and middle-aged computers, I can recommend that you install from an Ubuntu 64-bit 'amd64' iso file.



• 
  

  If you intend to use the external drive in old computers (as well as newer computers), I can recommend that you install from a 32-bit 'i386' iso file with an Ubuntu family flavour with a lighter desktop environment than standard Ubuntu,

  
  
  
  
  
  • 
    Lubuntu with ultra-light LXDE
  
  
  • 
    Ubuntu MATE with medium light MATE
  
  
  • 
    Xubuntu with medium light XFCE
  
  
  
  

• These 32-bit systems will work with 32-bit and 64-bit computers but only in BIOS mode, when installed according to the instructions [in this answer].



• 
  

  It is possible to create a persistent live system, that works in

  
  
  
  
  
  • 32-bit and 64-bit computers
  
  
  • in BIOS mode and UEFI mode
  
  
  
  

  If this is what you want, the following link may help you,

  
  
  

  mkusb/persistent#Compressed_image_file_with_a_persistent_live_system

  
  

Instructions

1. 
   

   When the computer is shut down and disconnected from the power grid, disconnect (and/or unplug) the internal drive(s).

   
   
   

   This makes the installer treat the external drive, where you want to install Ubuntu, like it were an internal drive (and the installer cannot tamper with the internal drive).

   
   


2. 
   

   Plug in the Ubuntu boot/live/installer drive (DVD disk, USB drive, memory card) and boot the computer from it.

   
   
   

   Proceed in your native language if you wish.

   
   


3. 
   

   Boot up the image

   
   
   

   The system boots properly and loads the installer displaying the Welcome
   dialog with language selection and 'Try Ubuntu' and 'Install Ubuntu' buttons

   
   



4. Connect the external drive (HDD, SSD, pendrive, memory card) where you want to install Ubuntu, the target drive. Plug in external power to this drive if possible. It might not be enough with the power from the computer's USB plug.



5. 
   

   Click on the Install Ubuntu icon

   
   
   

   The 'Preparing to install Ubuntu' screen is displayed

   
   


6. 
   

   On the screen Preparing to install Ubuntu, note the availability of the following components

   
   
   

   Available options should represent the state of your system accurately

   
   
   
   
   
   • (If network is available) Download updates while installing Ubuntu
   
   
   • (If on a 'laptop') Is plugged to a power source
   
   
   • Install third-party software ... option available. If you want the system to be portable between computers, please avoid third-party software, particularly proprietary drivers for graphics and wifi.
     
   
   
   
   


7. 
   

   Click on the continue button

   
   
   

   The 'Installation type' screen is displayed

   
   


8. 
   

   Select Erase disk and install Ubuntu

   
   
   

   Installation screen expands to include encryption and LVM options

   
   
   

   Wait a while! Are you sure that this is what you want? Maybe you want to keep something that is on the drive? In that case you should stop the installation and copy the important data to another drive. Maybe you want to select another alternative.

   
   


9. 
   

   Click on the continue button (if there is only one hard disk in the system, the button should read 'Install now')

   
   
   

   Write changes dialogue appears

   
   


10. 
    

    Click continue

    
    
    

    If there is only one hard disk, the installer skips to the "Where are
    you?' screen. Otherwise, the 'Installation type' screen is displayed

    
    


11. 
    

    If there is only one hard disk, skip a couple of steps to the 'Where are you?' screen. Otherwise, on the 'Installation type' screen verify that the drive selected on the Select drive list corresponds to the drive on the chart (e.g /dev/sda). If you have removed the internal drive(s), there should be only one drive, that is available as a target, your external drive.

    
    
    

    Selected drive is displayed on the chart

    
    


12. 
    

    Verify that the full drive space is allocated

    
    
    

    Full drive space is allocated for installation

    
    


13. 
    

    Click on the Install Now button

    
    
    

    The 'Where are you?' screen is displayed

    
    


14. 
    

    If your system is connected to the network, note the preselected timezone correspond with your timezone and the city indicated in the text box

    
    
    

    The timezone and city displayed match your timezone and the main city from your area

    
    


15. 
    

    Select your timezone, and click on the continue button

    
    
    
    
    
    • The 'Keyboard Layout' screen appears
    
    
    • The proposed keyboard corresponds with your keyboard
    
    
    
    


16. 
    

    Select your keyboard layout and click on continue

    
    
    

    The 'Who are you?' screen appears

    
    


17. 
    

    Input your initial user details and password. admin can not be used - it is a dedicated Linux User

    
    
    

    Name, username and password are accepted. Login options and home folder encryption choices shown

    
    
    

    Continue button becomes available

    
    


18. 
    

    Press continue

    
    
    
    
    
    • The 'Welcome to Ubuntu ' slide is displayed
    
    
    • The slideshow is entirely in your language
    
    
    
    


19. 
    

    Wait for the installer to finish

    
    
    

    An 'Installation Complete' dialog appears

    
    


20. 
    

    Click the Restart now button

    
    
    

    GUI is shut down, a prompt to remove media and press Enter appears

    
    


21. 
    

    Remove the disc and press enter

    
    
    

    The machine is rebooted

    
    


22. 
    

    Allow the machine to reboot

    
    
    

    The system boots properly and loads into Ubuntu showing the username that you selected

    
    


23. 
    

    Shut down the computer, unplug the external drives and unplug it from the power grid. Re-connect (and/or plug in) the internal drive(s)

    
    
    
    
    

    • If the external drive is an HDD or SSD, it is ready to be used now.

    
    
    • 
      

      If the external drive is a USB pendrive or memory card, it is a good idea to tweak the system to reduce wear. See the following link,

      
      
      

      help.ubuntu.com/community/Installation/UEFI-and-BIOS#Final_system_tweaks

      
      
    
    
    • 
      

      If you want a portable system (that works in most computers), you should think twice about proprietary drivers (typically for graphics and wifi). The classic advice is to avoid installing proprietary drivers, but it means that computers with certain hardware will not work well (or at all).

      
      
      

      In Ubuntu 18.04.1 LTS you can install an nvidia proprietary driver, that makes your computer with a powerful nvidia card use the full power of that card. The system will still select an Intel or Radeon driver, when booted in a computer with such graphics. But there will be problems with nvidia chips, that do no work with the installed proprietary driver. See this link,

      
      
      

      Install Nvidia drivers Full install USB flash drive

      
      
    
    
    
    

I did it using the following method:

• Insert live CD and plug in the USB key.




• Select Install Ubuntu.




• Chose Advanced when selecting drive partition.




• Chose your USB key partition as the target.




• CAUTION: Chose your USB partition for the GRUB bootloader.

After the installation process, boot on your USB key, not your hard drive.

1) Universal USB Installer:

Universal USB Installer is a Live Linux USB Creator that allows you to choose from a selection of Linux Distributions to put on your USB Flash Drive. The Universal USB Installer is easy to use. Simply choose a Live Linux Distribution, the ISO file, your Flash Drive and, Click Install. Other features include; Persistence (if available), and the ability to fat32 format the flash drive (recommended) to ensure a clean install. Upon completion, you should have a ready to run bootable USB Flash Drive with your select Linux version installed.

2) UNetbootin:

UNetbootin allows you to create bootable Live USB drives for Ubuntu, Fedora, and other Linux distributions without burning a CD. It runs on Windows, Linux, and Mac OS X. You can either let UNetbootin download one of the many distributions supported out-of-the-box for you, or supply your own Linux ISO file if you've already downloaded one or your preferred distribution isn't on the list.

3) LinuxLive USB Creator:

LiLi creates portable, bootable and virtualized USB stick running Linux. Are you sick of having to reboot your PC to try Linux ? No need with LiLi. It has a built-in virtualization feature that lets you run your Linux in Windows just out of the box.

All three programs above allow you to install any Linux operating system to a flash drive, but the persistence feature (allows you to save any changes made to a LiveOS installation permanent to be used even after reboot) is only available for Ubuntu and its many other flavors.

The only way I have been able to do it, is

• to burn the CD iso,


• disconnect my hard drive (physically remove cable(s)) and


• install to the USB.

Not very elegant, but it works.

You can, yes.

This process assumes you are installing from a live cd. While a live usb should work fine as well, the cd option is theoretically the safest, as there is no chance of overwriting the cd during the partitioning.

I recommend you start off by disabling your internal HDD in your BIOS first, as this makes sure there is no chance of accidentally overwriting your internal partitions. Also, the partitioning step of the Ubuntu setup will be much easier, since it will only detect the USB drive. With other words, it's best to make the USB drive the only storage device present on the machine during the installation.

Next, boot up the live cd and initiate the installation as usual. Make sure you choose "use whole disk" if you disabled all other storage devices, otherwise you will have to do manual partitioning.
In the last case, create an ext4 partition on the USB stick (make a partition table if there isn't one) and, if necessary, a SWAP partition if you intend to run heavy applications. Set the mount point to /. don't touch the other storage devices and their partitions!

When the setup asks for the bootloader location, choose the device name of your USB drive. This can be /dev/sda, /dev/sdb, so on, but don't choose a partition (e.g. /dev/sda1).

Wait for the installation to complete, then reboot. Make sure your machine boots from the USB drive. This can be made sure either from the boot menu (usually esc or a function key) or from the bios, where the boot sequence can be altered.

If everything went okay, Ubuntu should boot from the USB drive. If GRUB shows up, choose the first option.

Lastly, run the following command from your freshly installed Ubuntu desktop:

sudo chmod -x /etc/grub.d/30_os-prober

This makes sure that update-grub does not detect any other OSes that may be present on the system, as they do not matter for your Ubuntu USB drive installation. Also, when you boot up your USB drive from a strange computer, the OSes on its internal drive will be included into GRUB when a kernel/grub update occurs. This is unwanted.

Also, make sure to turn back on your internal storage devices from your BIOS.

The process of doing a full installation of Ubuntu to a USB flash drive is identical to installing Ubuntu on a hard drive except for the installing the GRUB bootloader. All of the steps to follow in the Ubuntu installer are identical except for installing the GRUB bootloader. Installing the GRUB bootloader on the USB flash drive will replace the existing GRUB bootloader on the hard drive which you don't want to do, unless you disconnect the cables to your hard drive(s) first before you start the Ubuntu installer to do a full installation of Ubuntu on a USB flash drive. So disconnect the hard drives first, then you can install Ubuntu on a USB flash drive.

You can alternatively do a full installation of Ubuntu on a USB flash drive without disconnecting the internal hard drive cables first by selecting the Something else option in the Installation type screen of the Ubuntu installer. You need to create only a single / partition (root partition), no swap partition, and change the installation of the grub bootloader to the USB flash drive. Double-check your partitioning choices that everything that will be changed is only changed on the USB flash drive, then click the Install Now button in the lower right corner.

Additional information about a full installation of Ubuntu on a USB flash drive.

• I recommend that the USB flash drive be at least 16GB.


• A USB 3.0 flash drive is a lot faster than a USB 2.0 flash drive.


• A swap partition will cause necessary read/writes to the USB flash drive, which will slow down the operating system and shorten the life of the USB drive. So select the manual partitioning option in the Ubuntu installer and create only a single / partition (root partition) without a swap partition. Change the installation of the grub bootloader to the USB flash drive before clicking the Install Now button in the lower right corner.


• The advantage of using a USB flash drive lies in its portability, not in its performance. The performance of a full install of Ubuntu on a 16GB USB flash drive is nothing like what you would get from running Ubuntu in a virtual machine application such as VirtualBox.

mkusb

Tested on Ubuntu 16.10 host, 16.04 USB, Lenovo Thinkpad T430.

Previously mentioned at: https://askubuntu.com/a/848561/52975 but here are more details.

Only available from PPA currently:

sudo add-apt-repository universe

sudo add-apt-repository ppa:mkusb/ppa

sudo apt-get update

sudo apt-get install mkusb

dus

and I tried the GUI version. More details at: How to make a persistent live Ubuntu USB with more than 4GB

I couldn't install NVIDIA drivers successfully however, bug report: https://bugs.launchpad.net/mkusb/+bug/1672184

qemu

Tested on Ubuntu 14.04.

Download the Ubuntu ISO.

Find your USB with:

sudo lsblk

sudo fdisk -l

Say it is /dev/sdX. Most often it will be /dev/sdb: sda is the main hard disk, and sdb the first USB. Now:

sudo apt-get install qemu

# Remove any existing boot sector, that causes installation problems.

sudo dd if=/dev/zero of=/dev/sdX count=16

sudo qemu-system-x86_64 -boot d -enable-kvm -hda /dev/sdX -m 512 

    -cdrom ./Downloads/ubuntu-14.04.2-desktop-amd64.iso

From inside the emulator, do a normal Ubuntu install that erases the old disk.

Installation took a bit longer than on a hard disk, but worked.

I tested with:

• plug the USB on a computer and boot from it


• create a file on my home directory


• reboot

The created file was still there.

Things have changed since 2011, Nvidia drivers are no longer required for Unity and unlimited persistence using casper-rw/home-rw partitions is possible, (but not with recent Ubuntu syslinux type boots, (SDC, Unetbootin, Roofis, Universal, etc)).

Mkusb is an installer that will fill all of the op's requirements, (if Nvidia drivers were only needed for Unity).

The user is given a choice of setup options and can select the percentage of space used for the persistent partitions, mkusb will then make remaining disk space available to Linux or Windows as storage. https://help.ubuntu.com/community/mkusb .

If the user does require Nvidia drivers a Full install is required as these drivers load before before persistence during boot

For Ubuntu 12.04 through 16.10 (all currently supported versions and flavors) the documented requirements vary but regardless an 8GB flash media should be sufficient to the task. A 16 GB version doesn't cost much more and can provide some "running room". The process itself couldn't be simpler.

1) Obtain a current ISO in the flavor of your choice

2) Check the hash to insure it's valid

3) Create a bootable media with the ISO (flash or optical disk)

There are a number of ways of doing this, my preferred method is to either

A) Flash drive method

Use dc3dd to simply duplicate the ISO to a target installer flash drive via the command line with the command sudo dc3dd if=yourisoname.iso of=medianame where yourisoname.iso is the name of the iso you downloaded and checked the hash for previously and medianame is the device name of your flash media. (as in /dev/sdb for example) you can easily determine the device name by checking the output of sudo fdisk -l

or

B) Optical Disk method

Burn the image to optical disk with your preferred OD image writing software. I'm rather partial to K3b but any optical disk burning software that supports the "Burn image" option should be suitable.

4) Continue to install normally as in:

Boot the installer and select the target flash drive as the target (I use manual partitioning AKA something else so that I can avoid creating a swap partition to reduce writes to the flash media that may cause early demise)

If any part of this process is unclear to you please drop me a comment and I will attempt to clarify. Note that if you are using a flash drive to install from, you'll need a second flash drive for your target installation.

EDIT: Another option would be to perform a Netboot Installation from the Internet I have not attempted this personally but include it as an option here in an attempt to cover all the options.

If you are careful there isn't much risk of overwriting the MBR of an existing drive in your system. After booting the live system you can run sudo fdisk -l from the CLI or Disks from the GUI and determine which drive is your valid usb target.

Once installation is complete you should be able to boot from your flash drive on any system with similar architecture simply by selecting your flash drive as the boot device in the BIOS.

It has been described here and in many other places how to install Ubuntu into a USB drive like you install it into an internal drive. It is straight-forward to do it with the standard installer (Ubiquity), if you can disconnect or unplug the internal drive. It works in either UEFI mode or in BIOS mode, the same mode as was booted when installed.

Installed system that boots from UEFI and BIOS mode

But if you want a USB drive with an installed system, that boots in both UEFI and BIOS mode, it is more difficult. I made such systems and prepared compressed image files, that can be installed in linux with mkusb directly, or in Windows in a two step procedure, extraction and cloning (and fixing the GPT). mkusb does it all, including fixing the GPT.

The mkusb Launchpad project is here.

The systems created from these compressed image files are quite portable between computers, not quite as portable as a persistent live system, but more flexible, when you want to update and upgrade the system (kernels, drivers etc).

See the following links and links from them,

help.ubuntu.com/community/Installation/UEFI-and-BIOS

Installed systems with guidus and gparted

You find compressed image files at this link,

phillw.net/isos/linux-tools/uefi-n-bios

Look for the newest files with updated versions of the installed program packages.

If there is a temporary problem to download these compressed image files, you can try the following torrent files,

dd_text_16.04-UEFI-n-BIOS_2017-01-15_intel-4-pendrive-7.8GB.img.xz.torrent

dd_dus-lxde_16.04-UEFI-n-BIOS_2016-12-12_intel-4-pendrive-7.8GB.img.xz.torrent

uploaded at UEFI-and-BIOS/torrent where you also find a short description. [Left]click on the torrent link, get to the attachment page, and there you right-click on the link and select 'save link as' to get the torrent file.

user: guru

password: changeme

dd_text_16.04-UEFI-n-BIOS_2017-01-15:

dd_dus-lxde_16.04-UEFI-n-BIOS_2016-12-12:

BIOS METHOD

Following is a step by step how to install 17.10 on a 16GB flash drive with options for separate Home partition and Windows compatible data partition:

• Create a live USB or DVD using SDC, UNetbootin, mkusb, etc.


• Turn off and unplug the computer. (See note at bottom)


• Remove the cover.


• Unplug the power cable from the hard drive or unplug the hard drive from the laptop.


• Plug the computer back in.


• Insert the flash drive.


• Insert the Live USB or Live DVD.


• Start the computer, the USB/DVD should boot.


• Select language.


• Select install Ubuntu.


• Select "Download updates while installing" and Select "Install this third-party software", (optional).


• Select "Continue".


• At "Installation type" select "Something else". (Full disk encryption is not working with flash drives).


• Select "Continue".


• Confirm target device is correct.


• Select "New Partition Table".


• Click Continue on the drop down.

(Optional FAT32 data partition for use on Windows machine)

• Click "Free space" and "+".


• Make "Size..." about 2000 MB.


• Select "Primary".


• Location = "Beginning of this space".


• "Use as:" = "FAT32 file system".


• "Mount point" = "/windows".



• Select "OK"




• Click "free space" and then "+".



• Select "Primary", "Size ..." = 4500 to 6000 MB, "Beginning of this space", Ext4, and Mount point = "/" then OK.

(Optional home partition)

• Click "free space" and then "+".


• Select "Primary", "New partition size ..." = 1000 to 6000 MB, Beginning of this space, Ext2, and Mount point = "/home" then OK.

(Optional swap space, allows hibernation)

• Click "free space" and then "+".


• Select "Primary", "New partition size ..." = remaining space, (1000 to 2000 megabytes, or same size as RAM), Beginning of this space and "Use as" = "swap area" then OK.

(Important)

• Confirm "Device for boot loader installation" points to the root of the USB drive. Default should be ok if HDD was unplugged.



• Click "Install Now".




• Select your location.



• Select "Continue".


• Select Keyboard layout.


• Select "Continue".


• Insert your name, computer name, username, password and select if you want to log in automatically or require a password.


• Selecting "Encrypt my home folder" is a good option if you are worried about loosing your USB drive.


• Select "Continue".


• Wait until install is complete.


• Turn off computer and plug in the HDD.


• Replace the computer's cover.

Note:
You may omit disabling the hard drive if after partitioning you choose to install grub to the root of the USB drive you are installing Ubuntu to, (ie sdb not sdb1). Be cautious, many people have overwritten the HDD MBR as default location for boot loader is sda, any items in the internal drive's grub will be added to the USB's grub.
You may do an update-grub later.

BIOS/UEFI Full Install

Mkusb makes a great base for many bootable pendrive projects, from grub2 bootable Puppy Linux to multiboot Persistent systems, multiboot Full systems and mixed/hybred Persistent/Full systems.

I used the following method to make a BIOS/UEFI Full install:

Use mkusb to make a Live system on a USB (2GB or larger).

Use mkusb to make a Persistent system on a USB 16GB or larger, using default settings with ~12GB persistence, (remaining NTFS partition is used as Windows accessible data partition).

Open GParted and delete sdb4, the ISO9660 partition and expand sdb5 into the recovered space.

Remove HDD before proceeding further, (optional but recommended, highly recommended in UEFI mode).

Boot Installer drive, select Try.

Insert Target drive

Start Install Ubuntu...

Select Something else.

Select sdb5, (on the target drive), and click Change.

Select Use as: ext4, Format and Mount point: /.

Don't touch any other partitions (unless adding a /home partition).

Select sdb5 for boot loader installation.

Complete installation.

Cut grub.cfg from sdb5/boot/grub and paste to sdb3/boot/grub, overwriting the existing grub.cfg file.

Boot the target drive and run sudo update-grub, (optional).

I figure this should work on any computer a mkusb built Persistent drive works on.

Please comment if it does not work for you.

Further discussion on creating drive using this method in UEFI mode starting at: https://ubuntuforums.org/showthread.php?t=2213631&page=17&highlight=usb post 169.

Another way to start Ubuntu installation is to get to Grub console from Grub2 Boot Menu.
It will fire up installation from Ubuntu ISO file on your HD.

Lets say you have your Ubuntu.iso on 3rd partition of your hard drive

Type c to get to Grub command promt and type the following:

loopback loop (hd0,3)/Ubuntu.iso

press "Enter"

linux (loop)/casper/vmlinuz.efi iso-scan/filename=/ubuntu.iso file=/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash

press "Enter"

initrd (loop)/casper/initrd.lz

press "Enter"

boot

press "Enter"

note (hdx,y) - is the partition where your Ubuntu ISO file is

This will fire up normal Ubuntu installation process just like you see it when installing from Ubuntu CD. From there you can choose your USB Flash Drive to install Ubuntu on it.

First prepartition your External drive.

Make a 500MB fat32 partition that is flagged as ESP (EFI system partition)
Make a 4GB swap partition.
Use the rest of the disk for your / partition. If you want separate /home and/or other partitions make them also.
gparted is good for this step but any partitioning tool will work.

When you install ubuntu, chose manual partitioning. Make sure the ESP and swap partitins on the internal drive and installer drive are set to "Do not use".

Assign the partitions on your external drive.
Then, Install ubuntu.

Now, external media requires /efi/boot/bootx64.efi in the ESP partition order to boot.

So copy /efi/ubuntu/grubx64.efi to /efi/boot/ and rename to bootx64.efi.

If you are using secure boot, /efi/ubuntu/shimx64.efi will need to be copied and renamed.

Note: you will need to temporarily remove the ESP flag in order to make this change.

you should now have a UEFI bootable external drive

I was able to do this using 2 USB's: one created as a USB Ubuntu installer the normal way (the installer USB), another to be the OS USB.

I recommend to remove your HDD's first.

1. Plug in the installer USB, boot into its live desktop


2. Run the installer, installing to the os USB

Worked a treat!

Now I've got an OS USB I can boot from anything! yay.

OP asks for "without touching my existing Ubuntu install."
If this means "don't touch my existing boot" and if you use UEFI, like any modern machine, you will run into a Ubuntu install bug that rewrites your boot partition on your internal drive no matter what you tell the installer.

Here's what works for me in Ubuntu 18.04, on a new laptop.
It's a Thinkpad T480, dual boot Win 10 & Ubuntu 18.04. I have turned off legacy boot. UEFI is 100% in use.

Installing onto a second drive is a pain because the ubuntu installer clobbers the first EFI partition it sees, which is the one on the internal hard drive, regardless of any attempt you make to specify an alternative location for the EFI partition. So when you try to set the boot partition on the USB drive, you are ignored. It's a fairly old bug. https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1396379

Summary: To workaround it, disable the internal EFI partition by using gparted to edit its flags immediately before beginning your install. Then the installer won't find it, and the bug is not tripped. Later, re-enable the flags. This is a trivial step.

The steps I took:

Boot into Ubuntu live USB in the "try first" mode.

Using gparted (you may have to install it first, sometimes Ubuntu doesn't include it on the live disk, although it is included in 18.04.1): ...

• re-partition your target external drive with a GPT partition table.



• Make a 500MB partition type FAT32. You may as well also set up the desired partition(s) for your Ubuntu install. You may find it handy to label the desired / partition because when you install you will have three drives: your internal drive, the live image installer drive, and your target drive.




• After applying those updates, change the flags on the small 500MB partition you just created. Right click on the small partition, and Manage Flags. These changes are actioned immediately (but note, you must actually create the partition first by completing the previous step)
  Tick to turn on boot, esp and hidden.

Before you start the install:

Edit the EFI partition flags on your internal drive. Untick those same three flags.
Now, when you install, the installer will see only one EFI partition, on your target device. This is the novel step which I haven't seen documented elsewhere.

Install

Use a live-disk USB image, as per a normal ubuntu install. So you have two USB devices: your target device, and the live-disk USB drive.

Proceed until you see the disk setup tab of the installer. You want the fully manual approach of course, "Something else" on the partitioning stage.

If your target drive is mounted as sdc and the EFI partition you made is sdc1, then you will be installing the boot loader onto sdc1,
Scroll to find that partition. It should say "efi" in the Type column. Click "change" to be sure: The installer should say "Use as: EFI System Partition". You won't actually be changing anything. No need to format it.

As you scroll through the paritions, review the Type column. There should be no EFI partition on your internal drive, since you turned off the partition flags on your internal drive EFI partition. You will see the EFI partition of the live disk you booted from to do the install, that's ok.

Mount your desired target partition for / and do a normal install.

After Install

After the install, reboot to the new installation. When you are happy things are working, you can restore the flags on your internal drive's EFI partition.

Now, reboot.
Choose your bios boot menu. You should see two Ubuntu choices in the boot menu, and one of them is the external drive.
Relabelling the EFI menu is a bit tricky. Make sure you boot into the installation on your external drive, then

sudo efibootmgr -v
Note the name of the file used to boot, and note the number of the partition.
my output for the first entry is:

HD(1,GPT,...) .... File(EFIubuntushimx64.efi)

and then have a look at this thread: https://www.kubuntuforums.net/showthread.php/68851-Labels-on-UEFI-Boot-Entries-using-efibootmgr-L

I did this to relabel mine:

efibootmgr -c -d /dev/sdb -p 1 -L owcUbuntu -l \EFI\ubuntu\shimx64.efi

knowing that the boot drive is sdb and since I made the EFI partition first, the value of the -p argument is 1.

Some "weird" things. Done right, this process won't touch your existing boot loader on your internal drive. However, when you have the external drive attached and are booted normally from your internal drive, grub updaters will detect the ubuntu installation on the external drive and add it to the list of bootable choices. This can get a bit confusing.

First of all, make sure that you boot your Installation Media in BIOS (Legacy) Mode, not in UEFI mode.

The problem you have is probably that you installed Ubuntu on your USB flash drive, but GRUB (the bootloader) was installed on the hard drive.

To fix that and install GRUB on your USB drive, you need to select the correct device when installing Ubuntu. When you get to the partitioning screen, you should have a drop-down list in the bottom (I can make some screenshots later if you need). There you can select, what device GRUB should be installed on. Make sure though that you select an entry without a number after /dev/sdx (x can be any letter). Assuming you installed Ubuntu on the /dev/sdc1 partition (you need to check yourself what the correct partition is), you should select the entry starting with /dev/sdc (Without any number behind it) in the drop-down menu. After that, continue with the Installation as usual.

I'll later add some information on how to revert the Hard Drive to it's old Bootloader.

EDIT: This seems to be a good start on how to restore your old bootloader (I'd recommend only the first two answers though) Uninstall Grub and use Windows bootloader

I followed the Ubuntu guide on their site.

Cononical recommend this program and I have uses it for every install of Linux to date and the program also writes Grub2 to the installation so all you need to do is select 'boot from USB' in your BIOS and go from there.

Here's the download link too: