Thanks for the advice! I tried all of the things you suggested and all seem to indicate no problem, but the single-user hangup persists. I'd hoped single-user-mode fsck might be a solution rather than another symptom.
The specific behavior I'm trying to fix is that a new mirrored RAID drive (WD MyBook Duo) that I use for data mounts normally, but later reappears a second or third time as phantom drives. It's connected as USB-C in the Thunderbolt port. Another single USB WD drive works normally, but it's a single-disc model. The problem seems to have started after I got an eSATA-to-USB 3 cable for an older FireWire hard drive. When I don't use that cable, the problem doesn't happen, but there's at least a day or more from when I attach the cable to when the phantom drives appear, so it's difficult to establish a real cause-and-effect relationship. After 10 days without the cable, no phantom drives, so I'm back to FireWire for a while.
Still, with no external devices attached at all, single-user mode still hangs. At this point I'm inclined to wipe the SSD and reinstall the system to help prevent problems down the road. It's sure nice to have Time Machine backups!
