Convert a Proxmox Windows Guest from BIOS to UEFI

Originally posted at https://blog.hadenes.io/post/convert-a-proxmox-windows-guest-from-bios-to-uefi/

In order to upgrade to a guest to Windows 11 or to passthrough a GPU the way it’s recommended in the official Proxmox documentation, you need to make sure the guest uses UEFI to boot.

By default, Proxmox uses SeaBIOS, thus chances are high that your Windows guest was installed in BIOS mode.

Gladly, in Windows 10, there’s a quick and easy fix to that. In this guide, I’ll walk you through the steps needed to convert your Proxmox Windows guest to UEFI.

This guide is based on the information provided in a Reddit post by user Browncow8.

Prerequisites

Before you begin, you’ll need to make sure that your Windows guest meets the following prerequisites:

• It is running Windows 10 version 1703 or later

Converting the Windows Guest to UEFI

Once you have the prerequisites in place, follow these steps to convert your Proxmox Windows guest to UEFI:

1. Open Command Prompt as an administrator in the guest machine.
2. Run the following command to validate the conversion process:

mbr2gpt /validate /allowFullOS

1. If the validation is successful, run the following command to convert the disk to GPT format:

mbr2gpt /convert /disk:0 /allowFullOS

1. Power down the machine.
2. Open the Proxmox web interface and navigate to the options for the guest machine.
3. Add a new “EFI disk” to the guest machine, making sure to enable the “Pre-Enrolled-Keys” option.
4. Add a new “TPM” device to the guest machine. (only necessary if you plan to upgrade to Windows 11)
5. Change the “BIOS” option from “SeaBIOS” to “OVMF”.
6. Change the “Machine” option to “q35”. (only if you wish to passthrough the GPU as a PCIe device)
7. Start the guest machine.

_Note: If you receive an error saying Failed to update ReAgent.xml, it refers to the hidden recovery environment and it should be ok to ignore it, or you can try

reagentc /disable

followed by

reagentc /enable

That’s it! You have successfully converted your Proxmox Windows guest to UEFI.

If you’re performing GPU passthrough to the Windows guest, make sure to follow the additional steps found in the Proxmox documentation at https://pve.proxmox.com/wiki/Pci_passthrough#GPU_Passthrough.

If you encounter any problems during the conversion process or with GPU passthrough, refer to the Proxmox documentation or seek help from the Proxmox community forums.