macOS High Sierra - Time Machine no longer recognizing iMac backups on USB drive My iMac backups that are stored on an external USB drive are suddenly not being recognized for backing up. Using option key and pull-down menu, I can select the set of backups on this USB drive and can restore from them, but I cannot get Time Machine to use that folder for more incremental backups — It wants to create a brand new backup (for which there is not room).

I have deleted the file /Library/Preferences/com.apple.TimeMachine.plist and rebooted the iMac, but the problem persists. I think the cause may be related to my very brief use of this USB backup file with Migration Assistant to put files onto an external hard drive: that drive now seems to be linked to this backup file in the Backups.backupdp folder.

How do I change that? I have discussed this at length with Apple support but was given no really useful advice.
The answer turned out to be very simple. In the command for tmutil I had copied the format and so I typed "[-a]" when I should have typed "-a". The error message always gave me the impression that the addresses were not correct. The proper error message should have said I should not include "[" or "]"