I have 5 cards hooked up to one machine right now. When I go to start all the cards up, the fifth card "[0-4] Cayman" always fails verification.
Disconnecting either the 2nd, 3rd, 4th or 5th card gets rid of the problem and the remaining 4 cards mine perfectly fine, meaning all the slots/cable/cards are ok, but adding a fifth card will make "[0-4] Cayman" ALWAYS fail verification, even when it's the only card mining and all other cards are stopped.
I'm using GUIminer 082011
What driver version, what OS?
The older drivers had problem w/ more than 4 GPU in Windows.