Virtual Machine fails to boot when changing the Firmware from BIOS to EFI

Products

• VMware vSphere ESXi
• VMware vCenter Server

Issue/Introduction

• Virtual Machine will fail to boot after changing the Firmware from BIOS to EFI or from EFI to BIOS. 
  
  
• VM will fail to boot with following error message on the Console : 
  
  

Environment

VMware vSphere ESXi 7.x 
VMware vSphere ESXi 8.x 

VMware vCenter Server 7.x 
VMware vCenter Server 8.x 

Cause

This is an expected behavior as changing the firmware is not supported since BIOS uses MBR (Master Boot Record) partitioning and EFI requires GPT (GUID Partition Table) partitioning on the VM disks. 


While changing the Firmware, a warning is also displayed that the installed guest operating system might become unbootable. 

Resolution

To make it work, we have the following options : 

 

• Rebuild the VM by choosing EFI/BIOS as firmware and attach the same hard disks to it to make it bootable.
• Convert the VM's disks to GPT before switching to EFI. This can be done by tools like mbr2gpt.exe in Windows and needs to be supported by Guest OS vendor, refer to the following article.