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I've set up a Samba share for Time machine backups according to this guide.

My Mac recognizes the share correctly, but the initial backup fails with the generic error message that no Backup could be created on my share.

Running log stream --style syslog --predicate 'senderImagePath contains[cd] "TimeMachine"' --info gives the following log:

MacBook-Air-2:~ xxx$ log stream --style syslog  --predicate 'senderImagePath contains[cd] "TimeMachine"' --info

Filtering the log data using "senderImagePath CONTAINS[cd] "TimeMachine""

Timestamp                       (process)[PID]    

2018-12-04 22:26:10.213195+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Starting manual backup

2018-12-04 22:26:10.338732+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Attempting to mount 'smb://pi@raspberrypi._smb._tcp.local./Time%20Machine%20Backups'

2018-12-04 22:26:10.897172+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Mounted 'smb://pi@raspberrypi._smb._tcp.local./Time%20Machine%20Backups' at '/Volumes/com.apple.TimeMachine.Time Machine Backups-74F8F302-361C-4490-9E35-1F2598F9D16E'

2018-12-04 22:26:11.816536+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Failed to find matching sparsebundle in directory '/Volumes/com.apple.TimeMachine.Time Machine Backups-74F8F302-361C-4490-9E35-1F2598F9D16E', directory contents: (

)

2018-12-04 22:26:11.845958+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Creating a sparsebundle using Case-sensitive Journaled HFS+ filesystem

2018-12-04 22:26:13.544412+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to create '/Volumes/com.apple.TimeMachine.Time Machine Backups-74F8F302-361C-4490-9E35-1F2598F9D16E/A76BCEEB-A3BD-5D63-82F4-8AEBB2F5ACBA.sparsebundle', results: {

}, error: 512 Fehler 512

2018-12-04 22:26:13.548494+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Backup failed with error 20: 20

2018-12-04 22:26:13.621842+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogInfo] Unmounted '/Volumes/com.apple.TimeMachine.Time Machine Backups-74F8F302-361C-4490-9E35-1F2598F9D16E'

2018-12-04 22:26:13.622367+0100  localhost backupd[34893]: (TimeMachine) [com.apple.TimeMachine:TMLogError] Failed to remove directory '(null)', error: 14 Bad address

The odd thing is: Contrary to the log a sparsebundle named 74F8F302-361C-4490-9E35-1F2598F9D16E/A76BCEEB-A3BD-5D63-82F4-8AEBB2F5ACBA.sparsebundle actually gets created in my share. So write permissions definitely aren't the problem.

macOS 10.14.1
Samba 4.8.0

It turns out that the configuration I tried does not work with HFS+ formatted drives. Reformatting the drive with ext4 makes the error go away so the backup actually starts.