Hard drive update:
I have a replacement, and have fitted it, but the drive that failed is still in the system. I haven't copied everything off it yet.
It's not particularly causing a problem other than its number of unreadable sectors reported by smartd continues to grow slowly (about 10 more every half hour, although sometimes it will instead inexplicably stay constant for a few hours)
The unreadable sectors were somewhere in the first partition on the drive. When the drive started to fail I made a second partition and copied everything into it. This second partition seems to be "away from the bad bit".
Data loss: I lost two of my rotated hardlink snapshots (you know, created with cp -al, rsync --link-dest, etc. rsnapshots) So I lost a few old versions of a handful of files. It hasn't been a problem. Everything else in the partition - larger downloads that weren't in /home as I didn't care about having backups of them - was fine.
The experience could have been a lot more traumatic. Still, you can't have too many backups. I still need better offline and/or remote backups.