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I have an external 1TB USB drive (mechanical, not flash) dedicated as a Time Machine backup. It is completely handled by MacOS/Time Machine.

Is it safe to unplug it at any time, including while a backup is running? I am aware how this stuff works generally (missing cached data, corrupting internal file systems data structures, journaling etc.), but would like to know if Time Machine is - together with journaling - robust in this regard for devices dedicated to it.

It would be acceptable to lose the currently running backup; I just want to avoid corrupting the filesystem or the Time Machine state on it so much that I have to reformat.

EDIT: Someone mentioned the "Golden Rule" of always ejecting. This question specifically asks whether this Golden Rule is relevant to Time Machine-managed devices, or whether Apple has gone out of its way to make a Time-machine-managed device immune to the usual problems of unplugging during use.

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    I agree that this is a valid question. I've run into many times where I needed to move my laptop quickly and Time Machine was keeping the disk busy. (Time Machine spends a lot of time before and after backups, and if you have it on automatic -- which it isn't very useful if you don't -- you can never tell whether you'll be locked for tens of minutes to not move your laptop, regardless of whether you try to stop the backup or not.)
    – Wayne
    Commented Sep 14, 2020 at 15:43
  • Hi, I have your same exact question. I often need to unplug my laptop quickly and forget to safely eject the USB drive I use for Time Machine. I'd expect Time Machine to be robust to this, in exactly the manner that you describe. Did you get an actual answer to your question, confirming or rebutting this? Thanks
    – a06e
    Commented Oct 28, 2023 at 10:58
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    No, I didn't, @a06e. tbh, I have long since stepped away from TimeMachine as my primary backup, and am running a custom rsync based setup now, which makes it very easy to have complete transparency about how everything's going. I accept that should something happen to the internal MacOS parts of my MBP I might to be in for a time-costly fresh install, but this gives me much more confidence for my own data.
    – AnoE
    Commented Oct 31, 2023 at 10:39
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    That said, @Maciej Miklas' comments on the existing answer make it seem rather hopeful that there should be little chance of destroying everything by plugging out at an unfortunate moment.
    – AnoE
    Commented Oct 31, 2023 at 10:41

2 Answers 2

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Golden Rule - eject a drive before disconnecting it.
Even if it doesn't appear to be busy.

999 times out of a thousand, you'd get away with it - it's just that one last time that will completely wreck your file system.

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    Yes, I know. This question is specifically asking whether this golden rule is relevant for Time Machine devices (i.e., if Apple has gone out of its way to make it 100% robust in this regard).
    – AnoE
    Commented Sep 14, 2020 at 12:52
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    It's relevant for any & all drives, no exceptions. There's no way to make anything robust against sudden disconnect.
    – Tetsujin
    Commented Sep 14, 2020 at 12:59
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    That's exactly what journaling file systems are about... they can absolutely be robust (not against loss of data that has not been journaled yet, but against catastrophic/total file system failure)...
    – AnoE
    Commented Sep 14, 2020 at 13:05
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    Then try it 1,000 times & see where you get.
    – Tetsujin
    Commented Sep 14, 2020 at 13:07
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    @MaciejMiklas - this has nothing to do with TM's checksumming, nor Journaling, both of which are designed to cope with this - but sheer bad luck, when the entire filesystem gets borked. Seen it far too many times to not take it seriously.
    – Tetsujin
    Commented Apr 26, 2022 at 11:31
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Apple says that yes, it can run when the drive is unplugged without ejecting but it is always best to eject it. If you're in a hurry, you can unplug it without ejecting as 9 times out of 10 thousand you will be fine, but again, always eject as the drive might be busy.

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