Random external drive unmounting and remounting on iMac

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Level 1

14 points

Random external drive unmounting and remounting on iMac

I have three external hard raves connected to my iMac (2020, 27 inch, that was upgraded to Sequoia 15.6.1 earlier this week). Subsequently, at random intervals one of the drive becomes unmounted and remounts automatically a few seconds later. As it is my 'Time machine' drive this is very frustrating. Further, earlier this morning all three external drives unmounted within a few moments of each other and the remounted.

I have read comments about dust in the connectors, and have checked for that. Nothing found and besides the drives are never disconnected so I think the dust hypothesis is unlikely. First aid run and not showing any errors

[Re-Titled by Moderator]

Original Title: Random external drive unmounting

Posted on Aug 23, 2025 08:39 AM

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Answer 1

den.thed

Level 10

88,750 points

Aug 23, 2025 09:27 AM in response to rob Burrows

Sounds like one or the combination of external drives is drawing to much power.

It can be a cable, a drive or the drive enclosure that is intermittently starting to fail and taking down the other drives.

Likewise if you are connecting two or more external drives to a bus powered USB hub, then that can cause power issues.

What I would do in an endeavor to troubleshoot the problem, is to only connect two external drives at a time and test.

For example connect,

The Time Machine backup drive and one of the other external drives, then test for a few hours.

Then the Time Machine backup drive and the other external drive, then test for a few hours.

Then the two other external drives w/out the Time Machine backup and test for a few hours.

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