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37 points

Disk Utility can't unmount external HDD

Since upgrading my iMac from Mojave to Catalina, DU won't touch an external HDD. It says it can't unmount the drive. Therefore, it won't run first aid, partition, erase, or, obviously, unmount. Trying operations from the command line (diskutil) yields the attached screenshot.

Also, the drive can't be unmounted by dragging it to the trash (Catalina reports "one or more programs may be using it"). Basically, the only way to unmount the drive is to shut down and unplug it before restarting.

By the way, while mounted the drive works perfectly for normal file operations.

Any ideas?

TIA,

Mark

Posted on Dec 14, 2020 10:22 AM

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Mark92630 Author

Level 1

37 points

Posted on Dec 15, 2020 6:18 PM

I still had problems, and further digging revealed that the underlying culprit was macOS "content caching" which is configurable under System Preferences .. Sharing. Somehow, macOS decided to implement content caching using a specific external HDD (which must have been mounted when I turned on that capability) to hold the cache. Once this happens, Catalina will never allow the HDD to be unmounted without a "force" option.

I moved the cache to the internal HDD, and now the external HDD unmounts easily, and can be operated on by DU.

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Disk Utility can't unmount external HDD

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