Weird. I just logged into my home server and noticed that the MD RAID array was degraded; turns out that it had decided that one of the drives was a spare and not part of the RAID array. I’ve no idea how that happened.
Rebooting fixed it; now the drive is back in the RAID and the RAID is rebuilding. Hopefully that won’t happen again.
I’ll also be interested to see what happens during the rebuild. The Linux kernel has been complaining about read errors on the active disk even though SMART hasn’t reported problems. If the rebuild goes OK, that should imply that the entire disk was readable after all.