VM fails to boot after changing firmware settings

Products

• VMware vSphere ESXi

Issue/Introduction

When a virtual machine (VM) is configured with EFI firmware, the Guest OS (Windows) typically uses a GPT partition table. If the VM firmware is switched to BIOS without converting the disk to MBR, the VM will fail to boot with errors such as "Operating System not found" or "No bootable device."

Environment

VMware ESXi 8.x / 9.x
Guest OS: Microsoft Windows 

Cause

The firmware mode (BIOS/EFI) must match the partition scheme of the boot disk (MBR/GPT). Switching firmware in vSphere settings does not automatically convert the guest's partition table.

Resolution

Prerequisites:

• Perform a full backup of the virtual machine or take a snapshot before proceeding.
• It is highly recommended to clone the VM and perform these steps on the clone first with the network adapter disconnected.

Step 1: Convert Guest OS Partition Table (GPT to MBR)

Before changing the ESXi settings, the boot disk within Windows must be converted from GPT to MBR.

1. Log in to the Windows Guest OS.
2. Use a conversion tool to change the partition style.
• Option A: Use third-party partition management software that supports non-destructive GPT to MBR conversion.
• Option B: Consult Microsoft documentation for native tools (Note: Windows native diskpart typically requires wiping the disk, which is not recommended for existing OS disks).
3. Verify the conversion is successful and shut down the guest operating system.

Step 2: Change VM Firmware Settings in vSphere

Once the disk is converted to MBR, update the virtual hardware configuration.

1. Log in to the vSphere Client.
2. Right-click the Virtual Machine and select Edit Settings.
3. Click the VM Options tab.
4. Expand the Boot Options section.
5. Locate the Firmware drop-down menu and change it from EFI to BIOS.
• Note: If Secure Boot is enabled, you must uncheck it before the option to switch to BIOS becomes available.
6. Click OK to save the changes.
7. Power on the Virtual Machine.