How can I create Windows install media that boots & installs UEFI ONLY?

Solution 1:

It turns out that it was in fact, quite simple, although I pieced it together based on information I found on the Microsoft website for Sys Admins.

The solution is to remove the bootmgr file from the root of the install USB. Note that the USB MUST be formatted as a single partition GPT disk in order to get a 4,1 or 5,1 Mac Pro to boot in UEFI mode. It may or may not work with other hardware that allows access to the EFI firmware, but if a Mac Pro finds an MBR, it boots in BIOS/CSM.

Furthermore, at least on my particular Mac Pro (as mentioned, a 2009 4,1 flashed to 5,1) it did not work when I formatted the USB drive using Apple's Disk Utility OR Gdisk - Sorry, Rod Smith. You are a legend and you have taught me much, but I had to use Windows diskpart to format the USB drive.

So, here is the exact step-by-step:

From any windows environment (I was booted in BIOS/CSM on my Mac Pro) open a command line prompt and type:

diskpart

list disk

sel disk *N* (where *N* is the number of your USB drive from the list)

clean

convert gpt

format quick fs=fat32 label="NAME"(where NAME is whatever you want to call your USB. Use quotes if you want to use a space, i.e. label="Windows 10")

exit

Then, still in Windows, I mounted the ISO I'd downloaded from MS and simply dragged everything EXCEPT the bootmgr executable to the root of the USB drive. Note that I did copy bootmgr.efi, just not bootmgr.exe.

Reboot holding alt/option, select the USB drive labeled "EFI Boot" and follow the instructions.

Per Stack Exchange policy, here's how I figured it out:

I ended up on this page. Where I found this quote in regard to booting in the desired mode, which is the EXACT problem I'm trying to solve:

If this is a frequent problem, you can remove the boot files for UEFI mode or BIOS mode to prevent the PC from booting in the wrong mode. If the PC firmware is set up to boot in the wrong mode, the media will immediately fail to boot, allowing you to immediately retry booting the PC into the correct mode.

Boot in UEFI mode: To prevent Windows PE from booting in BIOS mode, remove the bootmgr file on the root of the media.

Boot in BIOS mode: To prevent Windows PE from booting in UEFI mode, remove the efi folder on the root of the media.

They are talking about building custom Windows install media for deployment in a large organization, using the ADK and Windows PE, but I took at shot at this very simple solution and it worked for both Windows 8.1 and Windows 10.