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
ls -n does not show me that (501 is the logged user, and 20 is group
-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.