How Do I Get Rid Of (and Interpret) a Mystery HD?

Every time I restart my MacBook Pro (2015, latest Mojave), along with my actual Macintosh HD icon another icon literally named “Mystery” shows on the Desktop. It shows it to be a 1 TB drive, as is my actual drive. It shows it to contain almost the same, but not quite, amount of free space. When I open Mystery it shows zero items, though. I can readily unmount it with no *apparent* problem.
Any ideas for me? Might it be harboring bad things? I’d like it to go away forever.
Thanks.
—–
John Russell