Greetings. So, I have a Mac Pro 4,1 (5,1 firmware) with an NVIDIA GTX 980 and AMD 7850 installed, both of which are flashed. The problem is that whenever I attempt to boot without a monitor connected to 7850, the Mac Pro always freezes upon reaching the gray boot screen (prior to booting the OS) and the fans begin to ramp up. I can successfully boot into OS X by connecting a monitor to both GPUs but all attempts to boot Windows end in a black screen followed by a reboot. My intention was to use the 7850 for Mountain Lion and the 980 for Windows 10 but as far as I can determine, there is some kind of EFI conflict taking place. I have found that if I reflash the 7850 with its original PC ROM, the freezing on boot disappears. This would be a perfect solution but it also means that when booting Mountain Lion, the 7850 only initializes if there is a monitor connected to the 980. My only real recourse at this point is to purchase an active DisplayPort adaptor for the 980 to simulate a permanently connected display, so I would be very grateful for any input. Thanks!