As an experiment, I installed windows 8.1 on my mac pro 5,1 in EFI mode, its working perfectly and I have had no problems at all. I used the old ATI 5770 mac card to run this install since its an EFI card. When I tried putting my unflashed PC GTX 770 card in, win81 just goes to a black screen. I'm assuming it cant detect the video card. Anybody know a work around to get my PC GTX 770 working with win81pro in EFI boot mode?