Using tmutil compare to show the changes between the latest snapshot and the current state of the disk, I am told that most of, if not all, my files had their UID or GID changed. Here is one line from the output of tmutil compare as an example:

! 85.9K   (UID, GID)            /Volumes/A/path/to/file

But ls -n does not show me that (501 is the logged user, and 20 is group staff):

-rw-r--r--@ 1 501  20  88007 Dec  4  2017 /Volumes/MyTimeMachine/Backups.backupdb/MyMachine/Latest/A/path/to/file
-rw-r--r--@ 1 501  20  88007 Dec  4  2017 /Volumes/A/path/to/file

The two files look exactly identical but Time Machine thinks they differ. Why?!?

Background: I had a system disk, let's call it A, in my MacPro, with a Time Machine backup. For speed, I created a new system disk B on SSD, I moved some files from A to B but I will keep most of the data on A because B is too small. So now I would like to continue using the same Time Machine backup. The recommended way to do that is the command-line utility tmutil for that: I did

sudo tmutil inheritbackup /Volumes/MyTimeMachine
sudo associatedisk -a /Volumes/A /Volumes/MyTimeMachine/Backups.backupdb/MyMachine/Latest/A

It worked: I can browse the snapshots of disk A back and forth in time with the Time Machine interface. However, before starting a backup, I checked with tmutil compare what Time Machine thinks has changed on disk A, and that's the discussion I started this question with.

  • The current state of the disk and the last snapshot of it will always differ. There are constant I/O processes reading from and writing to your disk. – njboot Sep 2 '18 at 4:05
  • UID/GID have no reason to change, especially not on the scale I am seeing. There is something systematic going on but I don’t understand what! – frapadingue Sep 2 '18 at 10:01

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