Bdtyktl

Use:

sudo update-grub

sudo grub-install /dev/[BOOT PARTITION eg. Sda6]

commands in terminal of Ubuntu, probably it will solve your problem.

By this command system will automatically detect window installation and add them to grub list.

Sometimes os-prober is wrong.

Look at the UUID of your Windows 10 partition, for exemple:

sudo blkid /dev/sda2

Then edit /etc/grub.d/40_custom, and at the end of the file add:

menuentry "Windows 10" --class windows --class os {

   insmod ntfs

   search --no-floppy --set=root --fs-uuid $your_uuid_here$

   ntldr /bootmgr

}

Don't forget to change the UUID.

And finally update your grub config file:

sudo update-grub

Source

use this app EasyBCD for windows http://neosmart.net/EasyBCD/ download the free version and install it.

when you open it you will see an entry for windows 8 and one for windows 10.

delete the windows 8 entry and you'll be good to go.

Update after doing the steps above go back to Ubuntu,

open the terminal by pressing ctrl+alt+T

then copy past this commands to it one after another,

sudo add-apt-repository ppa:yannubuntu/boot-repair

sudo apt-get update

sudo apt-get install -y boot-repair

and by this you will install a tool called boot-repair.

open boot repair from dash and you'll get this window

choose the recommended repair and follow the steps.

Note you have to be connected to the Internet when using boot-repair.

This solution provides a proper entry in the grub2 menu and chainloads directly into Windows 10 without reference to the BIOS. It comes from https://ihaveabackup.net/article/grub2-entry-for-windows-10-uefi so I claim no credit. It was such a relief to find a working solution

Briefly, edit /etc/grub.d/40_custom and add:

menuentry 'Windows 10' {

    search --fs-uuid --no-floppy --set=root CC66-4B02

    chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi

}

To find the UUID for the --set=root line (CC66-4B02 in the example) you use sudo fdisk -l to identify the EFI partition then sudo blkid /dev/sda1 (or whatever) to find the UUID of the EFI partition. Note it's not the Windows partition but the EFI one you need. Once you've saved the edit, run sudo update-grub to generate the /boot/grub/menu.cfg file and then restart to test.

As far as I understood your question you have two different Windows installations on one or more harddrives and you don't want the Windows loader to get in the way or managing both Windows installations. I remember that it was possible on legacy setups to directly boot the Windows kernel from Grub (I may be wrong here) but I haven't been able to do this on UEFI setups. Having two independent Windows bootloader configurations that can be called from Grub independently should be very close to what you are looking for.

It came with Windows 8.1 installed.

I'm going to assume that this is a UEFI capable computer.

1. Please check that the computer actually is UEFI capable (I will remove my answer if it is not) and familiarize yourself with the differences between BIOS and UEFI (different boot modes, efibootmgr, different partition tables, EFI system partition (ESP), EFI loaders…).


2. Then check that all OSes are installed in UEFI mode and check that you have a GPT partition table. You can use Windows' dism to backup and restore partitions to a new partition table layout and use the command prompt from latest Windows installation media to reinstall the bootloader, more details on this in the next point.
   
   
   
   
   • Since you have been recommended to try various other tools that don't address your issue you have to revert these changes to a pristine condition, otherwise this will get more and more confusing. Sorry about that. :(
   
   
   
   


3. You can use my answer from How to boot Windows 8 from a legacy MBR partition in UEFI mode via GRUB?
   
   
   
   
   • This answer does install an independent Windows boot loader configuration through bcdboot in a different location where Grub will call it. For convenience I have chosen the Windows root partition. You will need to do this for every Windows installation, so that every Windows installation has its own and will only boot itself (clean the individual Windows boot menus from other entries).
   
   
   • Remember that you need to create an individual entry for each Windows installation you want to boot in /etc/grub.d/40_custom and follow the instructions and explanations about UUIDs, partitions and partition tables.
   
   
   • Probably disable every semi-smart feature that detects different OSes like OS-prober in Grub (GRUB_DISABLE_OS_PROBER) or troubleshooting boot issues with Windows or boot-repair (or manually clean up after them).
   
   
   
   

N.B.: Your request was to be able to manage all OSes with Grub, however with UEFI, the traditional "dualboot" term becomes almost a misnomer. UEFI allows coexistence of several bootloaders on the ESP and you can choose which one to boot (if they have been registered in the UEFI firmware and the manufacturers firmware doesn't do out of spec patronizing like only booting Windows). The sad thing here is that only very few people can think about installing more than one (independent) version of Windows, Linux or one release of Ubuntu onto one computer (or a harddrive that is used with several computers and boots a different installation of the same OS release on each computer). Most of this is already possible, but mechanisms in Windows and Ubuntu (Fedora,…) plainly choose to overwrite what is in their namespace under the false assumption that there can only be one.

(I do boot Windows8/10, Ubuntu, Ubuntu LTS and Fedora on one computer and have tinkered a few times with UEFI booting under different requirements.)

Just login to your Ubuntu OS, open a terminal:

sudo update-grub

reboot