Proxmox - Changing Windows 10 BIOS from SeaBIOS to OVMF (UEFI)

Even though Windows 10 is sunsetting in 2025 (on 14th of October), there's a lot of users that still use this OS as their main OS - whatever the reason may be. I am one of those users and heavily use this OS in my VMs on Proxmox.

As you may be aware, Windows 11 has much more stringent requirements for installation - using TPM is just one of those requirements. Of course, there are ways to install Windows 11 without using TPM, but officially, that is not a supported method.

On top of that, if you're using Windows 10 for business as BYOD, your company may require you to have certain settings the way they want, which is completely legitimate for BYOD's.

So, what happens if you happen to have Windows 10 that does not meet those requirements? Well, luckily, there is a way to convert Windows 10 using legacy BIOS (SeaBIOS) to OVMF without losing any data or a need to re-install Windows. It's quite easy actually, there are just a few steps involved.

Here are the steps:

• validate the conversion process: mbr2gpt /validate /allowFullOS - execute this command within an elevated command prompt (run it as Administrator) in existing Windows 10 installation; if the output of the command is successful, continue with the next step

• next, run: mbr2gpt /convert /disk:0 /allowFullOS - also in the elevated prompt
  • please note that in my case it was /disk:0, but in your case it may be a different designation, previous command (validation) will let you know what the disk designation is

• finally, in the Proxmox VM Hardware settings:
  • add new EFI Disk, make sure that option Pre-Enroll Keys is checked
  • add new TPM State device, making sure that its location is the same as previously added EFI Disk
  • finally, change BIOS to OVMF (UEFI)

Final result should look something like this:

And that's it. You should now be able to start the VM and if everything went well, you should basically see no difference.

I played it safe, so I actually created the clone beforehand, tested it in the clone and once I was satisfied it worked, I converted my main VM. I do have backups, but this was a way easier approach than to pull the VM back from the backup.