Time Machine partition won't mount
Okay, I figured out what was going on: When macOS detects a drive being unplugged without being ejected (which I don't think happened but I did get a message saying it had), the next time you plug the drive in fsck is going to go to work automatically, to check out the drive and attempt to repair it if need be. Unless you go looking, you'll get no indication that this is what's happening other than that the drive (or a partition on it) won't mount. This super helpful comment was key to my understanding what's going on: https://superuser.com/a/1013435/955941. After about 10 hours, fsck finished and the partition mounted correctly.