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Please treat your Mac to a UPS
WE LIKE TO think that utilities such as electricity are reliable, but know they’re not. If your Mac is any variety of MacBook, that’s no problem because it’s designed to switch to its internal battery in the event of wall power failure, and can’t take advantage of an Uninterruptible Power Supply (UPS) anyway.
But Mac minis, iMacs and especially Mac Pros die when the power stops. Good design now makes them less vulnerable to electronic damage, but some will only come back to life after expensive surgery, and all will lose unsaved data.
Your UPS doesn’t need to run your Mac for long, but it’s far better to have sufficient endurance on battery power for just a few minutes to shut everything down properly. A UPS is an extra expense but it could save you from data loss.
First Aid in Disk Utility can also report spurious errors when it fails to unmount volumes. Make sure that you read the transcript carefully.
> When a volume can’t be unmounted
What can I do when an app like Disk Utility reports that something failed because it can’t eject or unmount a volume?
This is an annoying problem which thankfully seems to be less of an issue with more recent versions of macOS. In Disk Utility the answer is to try again, several times if necessary, and it should eventually work. Double–check the volume that you’re trying to assess: if it’s one of the current startup volume group, such as the Data volume, you’re better off doing this in Recovery mode, although the error can occur even there.
If this persists, the free utility Sloth from bit.ly/ml189sloth is among several that will tell you which app or process is blocking the unmounting of that volume. Click on the padlock in its window and authenticate first so it’ll show all system processes being run as root as well as user processes.
In most cases, the process at fault is either the Finder, which you can restart from the Force Quit window produced by the Cmd+Opt+Esc keys, or Spotlight indexing, which normally finishes off if you wait a couple of minutes. A more radical solution is to restart altogether, but you’ll then have to wait for several minutes before all its startup activities have settled down.