could not unmount disk external hard drive

To discover which files are open on any volume, use the command Don’t just pull the cable of an external disk: not only will your Mac complain, but you could end up damaging the contents of that file, or even the file system on that volume. What you really want to know is which file(s) are open on the volume, so you have a clue as to what to quit to enable the volume to be ejected cleanly. When it has occurred in the Finder, solutions are harder to come by. It’s also worth double-checking the volume that you’re trying to run First Aid on: if it’s one of the current boot volume group, System or Data, then you’re probably better off doing this in Recovery mode anyway.

This seems to be improving steadily, and less likely to occur in Big Sur than Catalina, for example.

It’s unusual for this not to work at all, although it may take a couple of attempts. In Disk Utility, the answer seems to be to try again, several times if necessary. Not only can you encounter this in the Finder, but it can also prevent running First Aid in Disk Utility, which may report that it couldn’t because it was unable to unmount the volume you want checked. Among the rough edges in macOS is that infuriating message you may see when you try to eject or unmount a volume: it failed because the volume is in use.

Could not unmount disk external hard drive