I am working on my 2nd mining rig, this one is a little different than my last with 2 5850s, and 1 6870. When I first start the rig, all three video cards appear just fine in aticonfig --list-adapters. When I check with poclbm one of the 5850s doesn't appear, and when I go back to aticonfig only 2 appear. I'm not sure where to go with this one, any suggestions?
Check your power supply. It probably can't sustain what the BIOS deems necessary and takes it offline. I have a good quality 700W power supply and put my 6970 and 5850 in it and at first both worked. They were to close and heat was a problem (it was an experiment anyway while waiting for other hardware to arrive). Never again would both cards work at the same time (I use dual monitors so plugged both cards). It didn't matter whether I swapped slots, chose different power connectors, the 6970 simply will never start with the other card in and in fact it wouldn't work with my factory overclocked 570gtx. Running to close to the limits of your power supply can be tough on it and also your motherboard notices the variance and cuts a card. For me, it was the card with the highest power draw no matter the physical configuration.