Seems like my disks in the RAID array aren't failing, it's the usb top-loading enclosure tthat's holding the disks that flaked out... I think. The dmesg logs indicated a usb disconnect right before RAID got unhappy. And it was both disks, not just one, in contrast to what I first saw.
Anyway, backups are up and running fine again. Horray for backups!
@cwebber Hooray indeed!
(Suddenly thinking there should be a backup awareness week. 🤔 )
@cwebber What we really need is a World Restore Day.