Latest driver from AMD.com and the latest AMD APP SDK, latest guiminer, latest gpu-z, latest afterburner.
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...and you can't mine with a single 6990 core (i.e. any one of the four cores)?
Also are you attempting solo mining or pooled?
Right now Im just trying to get one 6990 to work properly (just one card in the mobo) and I can see just one core in guiminer if CrossfireX option es disabled in CCC, and I can mine ok but as i said just one core... If I enable CrossfireX option in CCC, I see both cores in guiminer but one core work ok and the second give "Verification failed, check hardware!" error.
Also in AfterBurner I see just one core with the right info, and the other core appears like no info... core speed, etc.