I updated from 10.10 to 10.13 (HighSierra) and I want to downgrade to 10.10. I have a backup on my TimeMachine from before the upgrade. When I restore my computer from TimeMachine, will it overwrite the new OS and replace it with 10.10 - or will only the files get replaced, but the underlying OS stays the same?

  • You might have to experiment, but if the TM drive has never seen 10.13 you should be able to boot to it & do a complete wipe & restore from there [you will lose anything since that backup unless you move it out to another location first]. If it's ever seen 10.13 it will not let you. See apple.stackexchange.com/questions/271104/revert-to-el-capitan for a similar trial & error.
    – Tetsujin
    Jan 26, 2018 at 13:33
  • @Tetsujin Oh dear... I updated to highSierra and my macbook (mid2012) is stupidly slow now... Then I have to find another way how to downgrade *edit: You wrote in the linked post that your TM synced with highSierra, yet you were still able to restore from a backup that wasn't done under highSierra. That contradicts with what you wrote here?
    – four-eyes
    Jan 26, 2018 at 14:24
  • @Stophface If you read Tetsujin's Q&A again you'll see he wasn't just using Time Machine as part of his backup regime, but also Backblaze and Carbon Copy Cloner. My interpretation from his answer is that he used Carbon Copy Cloner to make a clone of his drive prior to upgrading macOS and then when he decided he wanted to downgrade, he restored from that clone instead. He then manually moved across anything he may have needed from his other backups for the two week period since the upgrade (and clone) was made.
    – Monomeeth
    Jan 27, 2018 at 0:38


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