Time Machine: error occurred while enabling ownership on the backup disk Time Machine started failing on an external USB drive.


  Time Machine couldn’t complete the backup to “ExternalTimeMachine”.
  
  An error occurred while enabling ownership on the backup disk. You may
  need to repair or erase the disk using Disk Utility. If the disk can’t
  be repaired, select a different disk for backups.
  
  Latest successful backup: January 26, 2020


I ran First Aid from Disk Utility and there are no problems.

I tried enabling ownership manually. The Ignore ownership on this volume option is missing for this Time Machine volume's Get Info in Finder whereas other volumes on the same physical drive have this option. I enabled ownership through the command line and Time Machine continues to have the same error.

$ sudo diskutil enableOwnership /dev/disk5
File system user/group ownership enabled
$ sudo diskutil enableOwnership disk5
File system user/group ownership enabled


Disk Utility reports the following:

Mac OS Extended (Journaled, Encrypted)
Type: CoreStorage Logical Volume
Owners: Enabled
Connection: USB
Device: disk5
I removed the backup from Time Machine and added it back. It continued the backup successfully after that. Looks like yet another brittleness in Time Machine.