Windows 10 won't boot without resetting P-RAM

[Zeke D]

So, I'm running this cMP 2010:
2x hexcore 2.93 Xeons
32GB RAM (4x 8GB)
MSI TwinFrozr GTX680 2GB (flashed with the evga mac bios)
256GB SM951 AHCI (slot 2) [Sierra]
128GB SM951 AHCI (slot 3) [Windows 10 Pro N v1709]

I installed win10 in EFI mode, and it mostly works. I managed to find all the drivers from Boot camp 6 & 4, as well as manufacturer websites. Here's the issue:

If I reset the P-RAM, windows 10 will boot, and it'll be fine through warm and cold reboots. If I use the BootCamp program to "boot into OSX" Sierra boots just fine, but if I select win10 via the boot selector, or "startup disk" I get an error message saying something like boot drive not found. resetting the P-RAM is required to boot into windows10 now, and seems to be the default drive.

Thoughts?

 

[Senior-Switchmonkey]

Can you boot using ‘Option’ key on boot?

 

[Zeke D]

Can you boot using ‘Option’ key on boot?

nope, if I've selected OSX via the BC menu, then try to boot to the W10 disk via the boot selector, I get a single cursor in the upper left corner. reset the P-RAM, and it boots automatically into W10.