Everything is working just fine. If I click on [krocnyc’s] iMac in Finder, a hard drive icon named “lil” now appears where “Macintosh HD” used to be. Clicking on that takes me to my Applications, Library, System, and Users folders, which are all as they should be.
Does the name “lil” mean anything to you? Looks like something added a new volume to your APFS disk drive and is now sharing a container with your internal data volume. I can’t imagine how that would have happened by itself without some action on your part…
Many thanks, @karlnyhus! As I said, everything is working just fine, so I think that all that’s happened is that “Macintosh HD” somehow got renamed “lil,” although I can’t for the life of me figure out how.
I suspect that my problem’s origin just some fat-finger inadvertency by which my internal hard drive got renamed “lil.” Anyway, I changed “lil” back to Macintosh HD in disk utility, rebooted, and everything seems OK. (Yes, I was tempted to change the name to either “Magill” or “Nancy” …) I’m now on “Container disk5”; why the system blew right by Container disks 3 and 4 is a rabbit hole I just don’t have the energy for right now.