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?
QThis is an annoying problem which thankfully seems to be less of a problem 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.
AIf this persists, the free utility Sloth from bit.ly/mac372sloth is among several which 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 ç+å+oe 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.