I'm trying to install Windows 8.1 in UEFI mode on my Mac Pro 4,1 flashed to 5,1 with 3.46GHz Intel W3690.
As far as I can tell, Windows will only create media that boots the way the box that creates the media is booted. If the box booted via BIOS/CSM, the media it creates will (only) install Windows in BIOS/CSM mode. If the box booted via UEFI, the media it creates will install Windows in UEFI mode. Maybe someone can correct me if that is not the case.
All the instructions I've found around the web for creating UEFI install media involve selecting UEFI mode in a PC's BIOS screen and creating the install media. Of course, Macs don't have that option.
Here's what I've done:
I initially ran into the fairly well documented "select the CD rom type issue", but worked around that by following the instructions here and I successfully created a Windows 8.1 DVD using a PC, an ISO downloaded from Microsoft, oscdimg and ImgBurn.
I installed Windows 8.1 and upgraded it to Windows 10. You'd think I'd be happy and move on, but I have to make things difficult. :p I want to be booting from (U)EFI, not BIOS/CSM.
I formatted the target drive as a GPT drive according to these instructions, but when I try to install to it, the Windows installer (booted in BIOS/CSM) tells me that it can't install to a GPT formatted drive.
I successfully hacked my Bootcamp Assistant Info.plist and have the option to create a USB drive to install Windows 7 or later using Bootcamp Assistant. BCA reports copying Windows files, downloads the Bootcamp files and then simply reports that the install could not be completed. o_O
However, the USB drive appears to have all the necessary files to install Windows, just not the bootcamp files. I have bootcamp files from a separate download and they installed without issue to the BIOS/CSM version previously mentioned.
No matter what method I use to create the USB - Bootcamp, Rufus on Windows or by manually building it Windows - the USB shows the now familiar blue windows for a few seconds, then goes to a big blue screen and tells me:
Your PC needs to be repaired. An unexpected error occurred. A needed file is missing or corrupted \Windows\system32\boot\winload.efi Error Code: 0xc0000001 Use your install media to repair your PC or contact your Sys Admin...
My Mac Pro shuts down after 30 seconds or so.
I am very comfortable and well-versed in using the command line. I have Rod Smith's gdisk installed. I have XCode installed. I have Rufus and ImgBurn on the Windows side. I also have the Windows ADK, but I have not gotten into it because it seems to assume that you want to set up a network share to install Windows. If there's a way to create this media with that Windows tool, please provide detailed step-by-step instructions and remember that I cannot simply choose to boot in UEFI mode because my Motherboard does not have that option.