Video card works as secondary, but not primary?

[BigBadBiologist]

I'm having a weird issue with an AMD HD7850. If I install it alone (tried in 5 systems with same result), the computer hangs at POST as if there were no video card installed. If I install it as a secondary card with a HD 6850 as the primary card, the system can use it. I can even switch to the 7850 as the primary card in windows and run 3d mark on the card totally fine.

Anyone seen behavior like this and/or have ideas how to fix?