I'm attempting an EFI install of Windows 10 on a 2009 (4,1) Mac Pro firmware upgraded to 5,1. I'm running High Sierra, and installing Windows 10 on a second partition on the same disk. I have no problem installing Windows 10 via bootcamp (which defaults to legacy mode). In this case, the Windows Bootcamp Drivers (v. 5261) install perfectly and Windows boots normally (i.e., when holding ALT during bootup, I can select a partition labeled "Windows." I am also able to boot directly back and forth b/w Windows and Mac using Bootcamp Control Panel in Windows 10 / "Startup Disk" in High Sierra).
The problem is that I cannot seem to properly install Windows 10 in EFI mode. In order to install using the EFI option on the Win10 Install DVD (or a USB installation), I first have to create a partition (using Bootcamp Assistant or Disk Utility) and then reset my MBR to "Protected" wth Gdisk. I can then perform an EFI installation of Windows 10, but when I run BootCamp's drivers (I've tried BootCamp 5.1 v. 5261, as well as 6.1 and 4.0), Windows crashes with a black screen and then turns off.
Additionally, when I perform an EFI installation, I cannot boot into Windows XP from High Sierra using "Startup Disk." Although "Windows" appears in the last of startup options, if I reboot from "Startup Disk" I get a black screen with a flashing DOS cursor. With a legacy installation, I don't experience this issue.
I believe that Windows 10 EFI is not properly detecting my video card (an Apple NVIDIA GeForce GT 120). In Legacy mode, the bootcamp drivers properly register it as a GT 120, but in EFI mode, Windows recognizes it as a GeForce 9500 GT. With an EFI install, the only way I can boot fully is to first disable the Display Adapter in Safe Mode. Any attempt to install a video card driver (with Bootcamp or using NVIDIA's drivers) causes windows to crash and go into a boot loop until I disable the video card. Normal workarounds (like running BootCamp.msi from an elevated command prompt, or editing the launch conditions for BootCamp.msi using orca) let me complete installing Bootcamp in EFI, but I still have to disable the Display Adapter in safe mode, and I still cannot boot into Win10 from OS X (using "startup disk").
I've tried editing Bootcamp's Info.plist (to enable Windows 10, USB, and edit the 'PreUEFI' section) but this doesn't make a difference. I've also tried using WinClone to convert a (working) Legacy Mode windows installation into an EFI installation and I get the exact same errors (a bootloop until I disable the display adapter in safe mode).
I would keep the Legacy Mode installation and forget about the EFI installation - but my goal is to create additional partitions so that I can eventually triple-boot macOS/Windows/Ubuntu (which I cannot do with a Legacy Mode installation, which limits me to 4 partitions per disk). I would also like to avoid using alternative boot managers (like reFInd) and stick with the Apple Boot Manager instead.
If anyone had any insights, I'd really appreciate any help in getting my EFI installation to work (or, alternatively, adding partitions to a legacy installation).