Aha! You're getting there.
Maybe there is something funny about that disk. Did you replace it to cure the problem or just remove it from the server?
Another more distant possibility is that the power supply in the server itself is not quite up to powering two disks plus everything else whereas one is fine.
Or it could be a heat problem as you say.
Try to eliminate these one at a time (maybe run with both disks but the side of the server off to eliminate the heat possibility) until you nail it
