I've had a 6850 in my system before I learned of bitcoin. My new 6870 just came in.
I'm using GUIminer and it only recognizes both cards if they are crossfired. However, when I try to mine this way, it only gets about 230MH/s which is only 20 more than my 6850 alone got.
If I disable crossfire or take off the crossfire connector, GUIminer doesn't recognize the second card at all. What do I do? I feel like I should be getting well over 400MH/s now. Please help.
I currently have the same problem with 2x 5770. My board doesn't support crossfire but they should
work on their own.
Apparently you can try either a) changing something in your registry or b) connect another monitor or a DIY dummy plug
I'm rather pessimistic about my case because I can't even find the second card in the device manager or anywhere else. The fan is active but that's about it. Maybe not enough power from the supply or something.