Force Time Machine Full Backup on Incomplete Backup

I have an iMac (Retina 5K, 27-inch, Late 2015) running macOS 12.7.1 Monterey and the Finder indicates that my two separate (NOT Fusion) internal drives, have a total of about (3.75 + .091 =) 3.8 TB used.

I created a new Time Machine (“TM”) destination disk on an old USB drive. It was taking such a long time to run that I stopped the backup to see if something was wrong and concluded that I must have been using an old cable that was limiting data transmission. Then when I re-started TM, it appeared to backup only incremental changes in files because the Finder’s Get Info for the (destination) TM drive says that it has only 2.4 TB used out of 4 TB. I don’t understand why the used space on the TM drive (2.4 TB) isn’t at least as large as the used space on the two internal drives (3.8 TB).

Now that I’ve replaced the cable with one rated for USB 3, I want to force a full scan of the (source) drives and back up all the files that are missing on the (destination) TM drive, possibly about (3.8 - 2.4 =) 1.4 TB.

I’ve read that the only way to force a scan is to use a new TM drive.

Isn’t there a way to force a comparison of the source and destination drives that will add the missing 1.4 TB?

Thank you for your help.

There is no way to force a full backup, other than to erase the TM drive and start over.

However, an incremental backup will back everything the initial back up did not.

The Time Machine backup will always be smaller than the source, as logs, caches, and other non-essential files will not be backed up.

It used to be that you could force a Time Machine deep traversal by deleting the fsevents queue, but I do not think that works anymore – I can’t even find the folder.

I’ve noticed that it seems to be doing a deep traversal after a restart. I’d try this:

  1. Eject the Time Machine volume.
  2. Shut down iMac.
  3. Restart iMac.
  4. Reconnect the Time Machine volume.

Even better is if you can connect to the Time Machine drive to another computer in-between steps 2 and 3. Not to run Time Machine, just to get the last mount date/time changed.

Another thing you can do is to use tmutil to run a comparison of the last time machine snapshot to the current drive status, that will tell you if anything is missing. The hard part here is finding accurate instructions; a lot of the procedures are for Time Machine on HFS and don’t work on APFS.