Okay so I think my backup drive is failing. Got this in the mail this morning:
10 ? Aug 10 root ( 14) SMART error (CurrentPendingSector) detected on host: vile 11 ? Aug 10 root ( 14) SMART error (OfflineUncorrectableSector) detected on host: vile
In syslog, every half an hour smartd is reporting a new number of unreadable sectors: 2, 3, 9, 21, *, 11, 23, 35, 45. At *, smartd ran a scheduled self test instead, and didn't report any sector counts. The test appears to have reduced the count, but otherwise it increases each time.
I don't understand all of this obviously but I guess the drive is slowly failing. I wonder how long it has left...