I have done some research here, but most posts about this subject are old. The issue I am having is related to the use of two AHCI controllers, one on PCIe #2 and the other on PCIe #3. #2 = OS-X boot SSD #3 = Win 7 boot SSD Sometimes booting on #3 via option key works, sometimes it doesn't. I am not sure why it boots perfectly fine in #2 all the time, but not all the time in #3. Is there any known issue in the current EFI version (Mac Pro 5,1) not being stable on this kind of setup? The AHCI controllers are Apricorn x2 boards. The SSDs are Samsung 840 Pro. EDIT: I found out that windows will boot fine only when I choose to shut off the computer via OS-X first. If I choose restart instead, in OS-X, and try to boot in Win using the option key as usual, it doesn't work. I have noticed that when shutting down via OS-X, the white EFI screen that follows shows the spinning gear wheel before the computer turns off. Does OS-X write anything on the EFI when it is shutting down the computer? Windows always boots fine after this. Otherwise all I get is a black screen after choosing Windows via option key during boot.