Ouch. How much more textbook can you get?

Have 4-drive RAID server as main storage (and staging post for long-term backups - yes, RAID is not a backup, except temporarily). So last week I got a couple of SMART notices of recoverable errors on one of the disks. Time to replace it... Get new disk, pull old - but get bitten by the usual inconsistent mapping of logical to physical drives, so pull the wrong one. And having a brain fart, put it back. Rebuild triggered.

Oh well, the failing disk hasn't failed yet, so just have to wait for rebuild to finish before replacing the right disk. Except... Yes, you know where this is going... At "96.7% complete", there is a non-recoverable bad sector and the second disk gets failed out of the array, with rebuild of array not completed.

OK, now running ddrescue to recover as much as I can from the disk with bad sectors before I do anything else...