Windows – How to restore UEFI on a Windows 8 preinstalled laptop

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.

Ordinarily, I would be happy using UEFI, but at the moment I'm playing around with Linux live USBs, which will only show up in CSM mode (they do work in CSM mode).

I can't be 100% positive of this, but I suspect this is a case of the Linux USB drives being improperly prepared. Most modern Linux distributions support booting in EFI mode, including booting their installation and emergency tools in this way; however, some tools, like YUMI, omit the EFI boot loaders when they convert .iso files to USB boot media. The solution to this problem is to use another tool to prepare the boot media, or possibly to adjust options in the tool you are using. See this page of mine for more on this subject.

If you're using an old or obscure Linux live medium that lacks EFI-mode boot support, you may be able to add it yourself by installing a suitable boot loader and configuring it manually. This is likely to be a hassle and reqires a deeper-than-average understanding of Linux boot loaders. My Web page on this subject may be helpful, but you'll need to read at least the first couple of sections and the section(s) on the boot loader(s) you intend to use. You'll also need to be able to parse the medium's existing boot loader configuration so you can "translate" its configuration to your new boot loader.

Another option is to switch boot modes whenever you want to boot the Linux media. How you do this varies from one machine to another. In most cases, you can permanently set an option in the firmware so that it supports both EFI/UEFI and BIOS/CSM/legacy booting. With that done, the computer's built-in boot manager should show options to boot Windows or from the USB drive. (The built-in boot manager is usually accessed by pressing Esc, Enter, or a Function key at boot time; but details vary from one computer to another.) Alternatively, you could install my rEFInd boot manager and edit its refind.conf file so that scanfor is uncommented and biosexternal is among its options. This should cause rEFInd to show options for booting USB drives in BIOS/CSM/legacy mode, if they're inserted when you boot. (In some cases, you may need to press the Esc key in the rEFInd menu to get these media to show up, or uncomment the uefi_deep_legacy_scan option in refind.conf.)

One final comment: If Secure Boot is enabled, this feature can limit your options. Major Linux distributions like Ubuntu, Fedora, and OpenSUSE all support Secure Boot; but more obscure distributions might not. Likewise, if you install your own boot loader or use rEFInd, you'll have to either disable Secure Boot or jump through extra hoops to get it working, as described on this page of mine. You can usually disable Secure Boot without enabling the CSM, but I've heard reports of EFIs that link the two options.