After some painfully long media checks and fsck's, I had the drives working perfectly fine in the backup empeg. Then, just for shits, I popped the drives and cable back into the "problem" RioCar and it booted up fine...

So, Tony's suggestion of temperature being an issue seems likely.... The cable could have been loose etc, as well.

I'll keep everyone posted (if anyone cares) if I can replicate the issue on either or both players.