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I have an external 500 GB HDD that I've used for Time Machine backups for three different computers:

  • An old PowerBook G4
  • A white MacBook
  • My current retina MacBook Pro

I now have less than 100 GB free on the backup drive, so sooner or later it's going to fill up. I know that when the disk fills up, Time Machine will start deleting the oldest backups in order to make room for the new ones.

My question is: will it delete the oldest backups only for the machine that it's currently used with (which is the retina MBP), or will it delete the PowerBook's backups since those are technically the oldest on the disk, even though they're from a different machine?

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Will (Time Machine) delete the oldest backups only from the current machine's backups (the retina MBP), or will it delete the PowerBook's backups since those are technically the oldest (even though they're from a different machine)?

Time Machine only deletes the oldest backup for the current machine. macOS utilizes the File System Event Store to log each UUID during the backup process. Your machine's UUID should match whatever backups have been done previously for that machine, and only that machine's events are handled. All other events are effectively ignored.

You'll need to thin them out by hand using this tmutil delete command:

  • Great answer! It's good to know the technically accurate reason for this behaviour. – Ricardo Sanchez-Saez Aug 17 '13 at 15:26
  • I can confirm this is correct: my drive recently filled up and Time Machine deleted the earliest backup from my current machine, not an older backup from a different one. – daGUY Jan 12 '14 at 3:03
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Apple doesn't seem to confirm this specifically in their documentation. I have also not effectively tried this.

But I am pretty confident it will only delete the old backups for the computer currently being backed up, and will leave the other ones alone. If a Time Machine backup drive has extraneous files, Time Machine will leave them as is even when the disk is full. I assume that Time Machine treats files from Time Machine backups of different computers in the same way as extraneous files.

  • Thanks for the downvote. Care to explain so I can improve my answer? – Ricardo Sanchez-Saez Aug 17 '13 at 17:00

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