So I'm trying to run Windows 10 on my 4,1 --> 5,1. This Mac Pro is flashed to the latest everything so it can boot into Mojave. Strangest thing is happening though. I cannot get Windows 10 to boot with my MSI RX 580. If I drop in the GT120 (removing the 580), it works just fine. Put in the 580, I get the chime, but it does not boot. I'm doing all the usual things--no other drives installed, no USB drives attached, etc. It just doesn't like my 580. I even tried different ports on the 580 to see if maybe some don't work in Windows. I know it has been a few years since I tried to do this on a cMP, but I don't recall having any issues installing Windows with non-EFI-flashed cards. Ideas?