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I want to do the opposite of this question: Clean Lion install, keeping Time Machine history?

I backed up my computer, erased the disk, and made a clean install of Yosemite. I want to make sure that the old (Mavericks) backup never gets automatically erased; if there isn't enough space on the backup disk I want the backup to fail, not overwrite the old one. How can I be sure that it won't be deleted? Would it be safe to move the Mavericks backup out of Backups.backupdb? Ideally, I'd be able to treat it as a second machine. In the linked question com.apple.TimeMachine.MachineID.plist is mentioned; it would be nice to generate a new one.

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  • Not an answer, but as a matter of course, I pull the old drive & add a new one at any major OS upgrade. Old one is then dead-cert ready to revert if needed.
    – Tetsujin
    Commented Dec 30, 2014 at 19:01

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You've already done it. By erasing the disk you changed its UUID, so TM already considers the new disk completely unrelated to the old one.

The only problem you may run into is that if you ever do a full-disk restore from one of your old backups, TM will patch things up so that the restored disk (which now has yet a third UUID) is considered the one that all the old backups were from. It will now be the new backups that it won't ever erase.

Which come to think of it is probably exactly what you want.

In case that's confusing, let's take a specific example. You have a disk with UUID#1, and in March you begin letting let TM make a bunch of snapshots with it. TM remembers that they're snapshots of UUID#1. Then in June you erase the disk, so that it now has UUID#2 and let TM start taking snapshots of it. TM regards your snapshots from June onward to be snapshots of a different disk from a different computer. It won't touch the March-April snapshots.

Then in August you do a full-disk restore from one of your April snapshots. Part of a full-disk restore is to erase the disk, so it now has UUID#3. But at the end of the restore, TM will patch up the backup so that the March-April snapshots now appear to have been of the disk with UUID#3. The May-June snapshots, of the disk with UUID#2, are now the protected ones that TM will not touch.

Personally, though, I'd let TM start backing up to a new disk. That makes it easier for you to keep track of which backup is of which disk, and reduces the risk that you'll lose the old backups to a drive failure. Drive failure seems to be the biggest threat to a TM backup. If you aren't using the old backups, you don't need to have them mounted all the time.

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  • Thank you for your detailed writeup, but it's not helpful in my situation, because my "new" disk didn't get a new UUID. I probably ran into a bug that caused the UUID to stay the same even after an erase—aside from a collision, that seems to be the only explanation. I have, as I suggested in the question, moved the backup out of harm's way, and things seem to be going smoothly. The beauty of Time Machine is that with hard links, each snapshot works as if it's a full copy of the volume. Again, "probably exactly what you want" is exactly what I want. I just ran into a bug along the way.
    – 0942v8653
    Commented Dec 31, 2014 at 1:00

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