2 Replies Latest reply: May 16, 2012 8:32 PM by Pondini
BlakeEiseman Level 1 (0 points)

Hi all. I have a MacBook Pro and I've come to the conclusion that I need to do a fresh installation to solve several problems I've been having. My question involves my Time Machine backup. I have a complete, current backup on a USB drive.


After I do the fresh installation, rather than copy everything from the backup (and possibly bringing the problems with it) I would like to just selectively restore certain things. I know how to do all this.


The question I have is, after I set up my new system, what will happen if I activate Time Machine and point it to the same USB drive that has my current backup? Will it erase the backup, or will it create a new backup alongside the old one (there's plenty of room on the drive) or will it simply append the backup, treating it just like a new checkpoint in the incremental backup? I guess I'm hoping for the third option, because then I would be able to go back into Time Machine in case I forget something or want to copy something from my old system, even after I finish setting up the new one.


Any help would be greatly appreciated.

MacBook Pro (15-inch Early 2011), Mac OS X (10.7.4)
  • Glenn Leblanc Level 6 (9,325 points)

    It won't erase the old backup, but it will create a new one. I'm not an expert on TM, but you can read up on Pondini't website on TM:


  • Pondini Level 8 (38,740 points)

    When you erase a drive, it gets a new UUID (Universally Unique IDentifier).  That's what OSX uses to keep track of drives, not their names.  So it's treated as a different disk.


    If you do a full system restore, or use either Setup Assistant or Migration Assistant to transfer the data from your backups, that leaves a "trail" that Time Machine uses to figure out what's happened, and "associate" the "new" drive with the old backups.  The next backup will be only an "incremental" one, of what's new or changed, and the new and old backups will all be treated as if they'd been done from the "new" drive.


    However, if you restore things "piecemeal," there's no trail.  Time Machine will make a new, full backup of the "new" drive.  You can still see and restore from the "old" ones, via the procedure in #E3 of Time Machine - Troubleshooting.


    But if you use the exact same short user names (home folder names), then you may be able to force Time Machine to "associate" the "new" drive with the old backups.  See #B6 in the Troubleshooting article for instructions.