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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 Sep 14 '20 at 15:43
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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 Sep 14 '20 at 12:52
  • It's relevant for any & all drives, no exceptions. There's no way to make anything robust against sudden disconnect. – Tetsujin Sep 14 '20 at 12:59
  • 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 Sep 14 '20 at 13:05
  • Then try it 1,000 times & see where you get. – Tetsujin Sep 14 '20 at 13:07

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