[runamok]

I saw that once in an array that was on 24/7 for years. 1 drive was failing so they shut it down to replace the drive (perhaps hot swap was not an option?) and almost all of the rest of the drives did not come up. Basically the heads stuck to the platters. Aka "stiction".

I would guess in your scenario something like that happened. Or perhaps trying to migrate tons of data to a new disk caused an issue. Just seems unlikely otherwise.