I recently bought a 4,1 single CPU and did many of the usual upgrades (X5670, 24GB 1333MHz RAM, SATA SSD, 802.11ac+BT4.0 card). My impetus for doing all of this is I had recently purchased a Seiki Pro 4K 40" monitor (note it is a proper SST 4K monitor, not one of their TVs) and my old 2010 Mac Mini couldn't do 4K at all. I almost bought a 2012 or 2014 mini but the fact that they can only do 4K @ 30Hz led me down the cMP rabbit hole. Overall I'm very happy with my decision. But here's the problem I'm having: I bought a Gigabyte Radeon 7950 GV-R795WF3-3GD rev 2.0. I was able to get it for about $150 and prefer the 3-fan design to the one fan on the reference card. Port layout is identical to the reference (2 mDP, 1 HDMI, 1 DVI). I did a ton of research about flashing it over at netkas and had no problems adding the Mac EFI to the BIOS and flashing it to the card. Did all of that on a windows machine and the card works perfectly. But my problem is I cannot get the Mac Pro to boot if my Seiki Pro monitor is connected to the card via DP AND the monitor is set to DP 1.2 mode. I hear the power-on chime and the fan speeds up. I get a garbled screen on the monitor and the MP doesn't boot. Behavior is similar if I used the second mDP port on the card, with the exception of the startup chime repeats like a stuck vinyl record (bingbingbingbing...). If I leave the DP cable disconnected, turn on the Mac Pro, wait for the chime and then plug in the DP cable, the Mac boots just fine and DP 1.2 is fully enabled once I log in (it's doing 4K @ 60Hz). Or if I change the displayport setting in the monitor menus to 1.1 everything works perfectly. The Mac boots, I see the boot screen, etc. But I'm limited to 30Hz. Once booted I could use the monitor menu to change the DP mode back to 1.2 and then I can jump to 60Hz but that takes about 10 button presses on the monitor each time--would get old fast. The DVI port works fine too--but of course with the same limitation of 4K @ 30Hz. But that only works if I do not also have it connected to mDP. If I use the BIOS switch on the card to go to the other position (which still has the stock BIOS and PC UEFI) the Mac Pro will boot just fine with the monitor set to DP1.2 and with the DP cable connected the whole time (just no boot screen and once booted the card uses the generic framebuffer). I've tried SMC and NVRAM resets. I've also tried four different displayport cables (including two certified 1.2 DP->mDP cables and a regular DP->DP cable with a DP-mDP adapter). Behavior is the same with all of the cables. Having done plenty of research on this beforehand, I was prepared for the fact that I likely wouldn't get a boot screen with DP1.2. But I was not at all expecting a situation where my monitor would actually prevent the Mac from booting period. I plan to try to contact Seiki to see if this is some kind of bug with their monitor. But I also wanted to post here in case anyone has seen this kind of issue before and had any ideas. Thanks! Edit: Unfortunately I don't have another 4K DP1.2 monitor available to test, nor do I have access to another 7950.