No video detection after adding second graphics card.
I had a completely working build with an Nvidia GTX 580 and I was using two monitors perfectly. I got a cheep ATI card from a friend and installed that as a second card alongside the Nvidia one because I wanted to see if I could get three displays working. When I tried booting the system, no video was recognized, but everything else seemed to be fine. So I removed the ATI card and tried booting with the Nvidia one again. Again, I can not get video to work. I tried booting on one stick of RAM and without the graphics card and everything seems to be fine, but I can't get any video.
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Try swapping in a know working PSU.
The PSU works, it turns out I needed to update/re-install the BIOS via USB drive. It is working now with one card again.