Mac – Does Time Machine change file permissions while doing a backup

The first question, how can you be sure a backup has completed, is answered by checking the menubar Time Machine item. It will tell you the date of the last completed backup.

Secondly if you want to perform another complete Time Machine backup you can stop Time Machine, eject the Time Machine drive, plug in a fresh drive and re-enable Time Machine using that drive as the backup destination. A complete new backup will be performed.

You could also just delete everything on the original Time Machine drive but it is slightly more risky. If something bad happened before the new backup completed you would be without the backup you are not sure about and the new backup.

You can safely restore files using finder or the command line (rsync or other) if you want to avoid using the Time Machine animation.

The reason why needs explanation: Technically, the system uses hard links. So if you make three new files on the desktop named A, B and C and then make three backups. This is what happens:

1. The first backup copies the new files to the correct location on the backup drive (in your user's home folder for the time specific snapshot - or /Volumes/Drive/Backups.backupdb/mac_name/2011-07-03-123456/Mac HD/Users/you/Desktop/A and B and C.
2. The next backup detects that the file is unchanged and makes a hard link in the next backup interval desktop directory pointing to the original file on the first backup.
3. Now modify file B and make a backup. There will be one copy of A, C - each with link count of three (three intervals have that version) and there will be two versions of B one version with link count of 1 and one version with link count of 2.

Now the file "exists" in both directories with a link count of 2. Any copy operation copies the correct bits to the new place, and if you deleted one copy (Time Machine prevents this with Access Control Lists) of file A it would decrement the link count on the file and remove it from the folder list where you deleted it.

Eventually when the link count of a file is 1 - deleting it deletes both the data and the entry for the file.

For your case - you could pick a time and copy whatever files you want as hard links don't cross volume boundaries and you'll end up with a non hard linked file with link count 1 where you put the file on your current disk.

You didn't ask, but be sure to use tmutil to clean up files on the Time Machine or the graphical interface. They understand link counts and work well. If you try to delete files manually (say dragging them to trash) - you are in for a bad time unless you are sneaky about it.