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.

  • Have you recently upgraded to High Sierra? Have these issues begun since then? Are you using the new Apple File System in High Sierra, or are you using the older file system?
    – Seamus
    Commented May 13, 2018 at 17:44
  • Further comment: It looks like this problem could be solved using the utility tmutil. Commented May 13, 2018 at 19:07
  • After I upgraded to High Sierra I continued to do automated incremental backups to this drive. I am using 10.13.4 from the start of my upgrade. I can extract backups from the file but I cannot incrementally add to it. The apple unit utility tmutil is intended to deal with exactly this problem of realigning backup assignments, but I am not able to get it to do this Commented May 14, 2018 at 16:19
  • When you upgraded to High Sierra, did you also upgrade the file system to APFS?
    – Seamus
    Commented May 14, 2018 at 16:28
  • I just ran the installation of High Sierra, using a downloaded installer from Apple. Whatever the installer did was done. I did not take any action to reformat any of my hard drives Commented May 15, 2018 at 18:04

1 Answer 1


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 "]"

  • What command were you using?
    – Mackey18
    Commented Nov 20, 2018 at 18:53

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