"Disk you attached not readable" - PMBR/MBR corrupted?
Editing sector 0 (the PMBR) of the bad drive to fix the corruption fixed the issue, at least for now. I can see/open/edit all files so far.
There's still a chance of hw failure, even though the disk passes all firmware checks and "disk repair" completes successfully.
I was lucky to have an identical disk which had been treated in an identical manner so I could use those values in the first sector.
Believe this may have been precipitated by a few power outages that hit this server in the weeks preceding the failure, but that's just a guess.