How to create an EFI bootable Windows installer USB stick w/o Rufus ?

Dear forum readers,

I downloaded the official 21H1 ISO from Microsoft. Unfortunately however, MS doesn't make hybrid ISO images, hence it won't boot when written to a USB stick. Some BIOSes provide CD/DVD emulation, but it generally doesn't work well, and fail at some point.

That's why I'm trying to change a few things to get the installer working on a USB stick. I know there are helper programs that will do the job such as Rufus or MCT, but there are reasons to try without such programs :

• Being on a non-Windows OS
• MCT doesn't produce installation medium in a reproducible hence verifiable way
• Remaining autonomous as much as possible, not relying on helper programs

I've read that things were as easy as formatting the stick in DOS/MBR format, creating one FAT32 partition and copying the contents of the ISO to the root of the partition. In case some files are too big (> 4096 MiB), it's most likely the one named install.wim, either convert it in ESD format (install.esd) or split the file in smaller SWM chunks (install.swm, install2.swm).

I did that, it didn't work. I tried with formatting in GPT format instead, with the partition type being either EFI or Microsoft Basic Data, no more luck.

I can see it starts booting, with some serious access to the USB stick, but it always ends up either freezing or rebooting. What am I missing ?

dalchina said:

Hi, have you seen Option 4?

Thanks, now I have a procedure that's deemed working !

I'm on Linux but know diskpart so I know what I should do. There are two things that differ from what I did :

• I didn't set the partition as active
• I converted install.wim to ESD or SWM, but didn't try to extract only one edition of Windows

Concerning the partition being active or not, it should not be relevant in a UEFI boot context, but who knows, maybe the Windows installer still gives it some importance.

Ah, there may be another difference : what is the type of the partition created by default on diskpart ? I don't remember that, and maybe that's the problem.

I will update this thread with my results.

NovHak said:

I downloaded the official 21H1 ISO from Microsoft. Unfortunately however, MS doesn't make hybrid ISO images, hence it won't boot when written to a USB stick.
[*]MCT doesn't produce installation medium in a reproducible hence verifiable way

Please explain the above sentences.

A flash drive > or = 8 GB is typically used.

The MCT is designed to format the flash drive before installing the ESD iso.

It should be bootable in computers using BIOS Legacy or UEFI.

Hi zbook, thanks for coming in.

I downloaded the official 21H1 ISO from Microsoft. => More precisely, it's the french 64-bit installer, from https://www.microsoft.com/software-download/

Unfortunately however, MS doesn't make hybrid ISO images, hence it won't boot when written to a USB stick. => I'm not sure what I should add. Hybrid ISO images are ISO images that are either in DOS/MBR disk format, or GPT disk format as well. It's not the case with the MS-provided ISOs.

MCT doesn't produce installation medium in a reproducible hence verifiable way => From what I've understood MCT builds Windows installation media by using files from a production system, so two MCT media created on different systems may give different results depending on the system's update status.

Concerning the size, I use USB sticks that are at least 8 GB (the one I currently use is 128 GB).

I don't use MCT for the three reasons I mentioned : I'm not on Windows, I want reproducible results, I prefer doing some things myself if possible, avoiding helper programs.

I'm not considering BIOS legacy environments right now, UEFI is enough.

NovHak said:

Hi zbook, thanks for coming in.

I downloaded the official 21H1 ISO from Microsoft. => More precisely, it's the french 64-bit installer, from https://www.microsoft.com/software-download/

Unfortunately however, MS doesn't make hybrid ISO images, hence it won't boot when written to a USB stick. => I'm not sure what I should add. Hybrid ISO images are ISO images that are either in DOS/MBR disk format, or GPT disk format as well. It's not the case with the MS-provided ISOs.

I'm afraid you are mistaken, @NovHak. The contents of the ISO file that you get from the Media Creation Tool is bootable on both legacy BIOS and UEFI computers. If you partition an 8 GB or larger USB flash using MBR with 1 partition formatted as FAT32 and marked as Active, then extract the ISO file to it (you don't copy the ISO file itself, you extract the contents of the ISO file), the USB flash drive will be bootable in both legacy BIOS and UEFI computers.

Some UEFI computers will only boot from a FAT32 partition, so that is why Microsoft formats their USB flash drives as FAT32 (and also why the extracted files and folders from an ISO file created by the Media Creation Tool will always fit in a FAT32 filesystem).

Legacy BIOS computers require the flash drive to be MBR with an active partition in order to boot from it.

So, you combine the above requirements with a flash drive partitioned as MBR, with a FAT32 partition marked as active and it is bootable in legacy BIOS and UEFI computers. That's the way Microsoft designed their Media Creation Tool to work.

See if these links are useful:

Download Windows 10 ISO File
https://www.microsoft.com/en-us/soft...load/windows10

NavyLCDR said:

I'm afraid you are mistaken, @NovHak.

I think there's a misunderstanding somewhere. I never said that the MS ISOs are unable to boot on legacy BIOS or on EFI, I know they are perfectly able to. The problem is that without any change, they will only boot if burned on a DVD (or through an IODD device of course). If "burned" on a USB stick as is, they won't boot because the ISO isn't hybrid.

zbook said:

See if these links are useful

I've seen them, and that's how I got the Win10 ISO (I used step three of the tutorial, except I didn't have to modify the browser User-Agent header to fool MS into thinking I have a non-Windows OS because it's in fact the case).

NovHak said:

I think there's a misunderstanding somewhere. I never said that the MS ISOs are unable to boot on legacy BIOS or on EFI, I know they are perfectly able to. The problem is that without any change, they will only boot if burned on a DVD (or through an IODD device of course). If "burned" on a USB stick as is, they won't boot because the ISO isn't hybrid.

NovHak, You don't burn a iso to USB drive. You extract the iso to the USB drive Fat32 partition
NavyLCDR gave you a complete explanation on how to make an USB Win 10 installation drive from a Win 10 installation iso.
Format the USB drive partition as Fat32
Make it active.
Extract the iso file files and folders to the USB drive Fat32 partition.

That is all you need to do.

Windows can be installed in two ways: Legacy-MBR or UEFI-GPT
To install as Legacy-MBR you must boot the installation drive as Legacy
To install as UEFI-GPT you must boot the installation drive as UEFI.

If your MB has a UEFI BIOS, you should install as UEFI-GPT, but if you already have Linux on the drive, you must choose the same drive architecture (Legacy-MBR or UEFI-GPT)
Detach any other drives (SATA or Power cable) from the MB.

During POST, press F12(?) to launch the boot menu. You will see two options for the USB drive. USB UEFI (Name) and USB (Name). Select USB UEFI (Name) for UEFI-GPT or USB (Name) for Legacy-MBR.

If you don't want to use MS account, don't enable updates or connect to the internet during installation.

NovHak said:

I think there's a misunderstanding somewhere. I never said that the MS ISOs are unable to boot on legacy BIOS or on EFI, I know they are perfectly able to. The problem is that without any change, they will only boot if burned on a DVD (or through an IODD device of course). If "burned" on a USB stick as is, they won't boot because the ISO isn't hybrid.

Well, again, you are mistaken. There is no ISO file that you can just copy the ISO file itself to a USB flash drive and boot from, whether it is a hybrid (whatever that is) ISO file or not. But if you extract the files and folders from the ISO file to the USB flash, it will make a bootable flash drive, both legacy BIOS and UEFI bootable if the flash drive is formatted correctly. The same is true for making a DVD. You can't just burn the ISO file itself to a DVD. You have to burn the extracted files and folders contained in the ISO file to the DVD. Most DVD burning software will extract the files and folders from the ISO file when you try to make a DVD from the ISO file.