The properties for the first drive in the tree, which I later found out happens to be drive D:, are shown.
Here is how I think it might have happened: my wife was having trouble with a 2Tb drive in her build that was randomly disappearing in the operating system -- she would open up Explorer and it would no longer be there, would just disappear without warning or explanation. Sometimes a reboot would bring it back. When the problem started getting worse, we removed the drive, thinking the drive was the problem. So, just to make sure that it was really the drive and not a motherboard or operating system issue, I put the same drive in this build, the one we're talking about in this thread, formatted it and let it run for a while. I put a Steam library on it and moved one of my games over just so that there would be some data on it. Never had a problem with it, drive worked just fine.
Some additional troubleshooting on her build revealed another problem: I noticed -- completely by accident -- that the 3V supply rail was in the red in BIOS. Reseated the PSU cables, that brought the rail back up to where it is supposed to be, and the problem with the drives dropping out has never returned.
(Segue to current thread discussion...)
When we got the two new 4Tb drives, to prepare for install them I moved the Steam game off of the 2Tb drive, shut the PC down and removed it, as it was in one of the two m.2 slots where the new drives were going to be installed. In retrospect, what I think I should have done was, before shutting down, gone into Diskpart and run a "clean" command on the 2Tb drive and left it in an unallocated state before removing it. It looks as though the Windows registry retains the drive information under the System key otherwise. Similarly, after the cloning was complete, I physically removed the two old spinner drives, and I probably should have done the same for those, as I still see them in the registry even though they are no longer physically there.
So this all comes down to sloppy administration on my part, unfortunately. I just wish that there was a way to clean that information up without the associated hardware being present.