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I just upgraded from El Captain to Mojave and I have a Time Machine backup of the instant before starting the upgrade.

I need to be able to eventually restore the previous operative system, but I am afraid that that backup could be removed in order to make space for newer backups.

How can I "mark it" and ensure that that specific backup will not be discarded?

  • once you have let Mojave back up to the TM drive, you have lost the ability to roll back to El Cap from it. See apple.stackexchange.com/questions/271104/revert-to-el-capitan – Tetsujin Feb 18 at 11:17
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    @Tetsujin Are you sure? I've rolled back to a previous OS several time using TM. Last time was c. Sierra, I think. – benwiggy Feb 18 at 11:42
  • @benwiggy - having spent quite some time trying to do it, pretty sure. How did you manage to do it, wholesale or just piecemeal? – Tetsujin Feb 18 at 14:37
  • @Tetsujin Boot to Recovery, choose Restore from TM, choose a snapshot. Bingo. support.apple.com/en-gb/HT203981 – benwiggy Feb 18 at 15:45
  • Flatly refused when I tried it. – Tetsujin Feb 18 at 15:50
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There is no way of marking a TM snapshot as being particularly important.

However, as long as the TM disk isn't full, then TM should not normally delete the snapshot that accounts for a complete OS change. (Assuming that you had the old OS on your disk for more than 1 month.)

TM will start deleting the oldest snapshots if the drive is full, to make space for the newest ones.

If you want to make absolutely certain, you would do well to use an app like SuperDuper! or CarbonCopyCloner to make a clone disk image, which you can restore at a later time. (But you needed to do this before upgrading.)

Bear in mind that when you roll the whole disk back, you will lose any changes to your data made since the rollback date. IMAP servers will handle changes to email, but documents and other files will need to be restored by other means.

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