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I am trying to convert a Windows 10 Enterprise VM from BIOS boot to EFI/UEFI. After extensive research I found the built-in "MBR2GPT" tool that MS released with Fall Creators Update: https://docs.microsoft.com/en-us/windows/deployment/mbr-to-gpt

However, when I boot to the Win10 ISO (so WinPE) and try to run the "MBR2GPT .exe" tool it fails:

E:\Windows\System32>mbr2gpt /convert 
MBR2GPT: Attempting to validate disk 0
MBR2GPT: Retrieving layout of disk
MBR2GPT: Validating layout, disk sector size is: 512 byes
Cannot find OS partition(s) for disk 0

I run DISKPART first to ensure the OS disk is indeed disk 0. I'm desperate for a solution here as we have almost 20 machines to image and are up against a hard deadline. Help greatly appreciated!

Here are my logs: https://drive.google.com/drive/folders/10Qytdhmr6JDyLndCfrfUGQrsYL2PVAKv?usp=sharing

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This is resolved now. I could not get mbr2gpt.exe to work BUT I was able to get GDisk to work (online documentation is bad, overly verbose and long winded, but just boot to WinPE and run the GDisk64.exe for command discovery): http://www.rodsbooks.com/gdisk/mbr2gpt.html

  • Download GDisk, copy gdisk-windows-1.x.zip to USB that has Win10 ISO on it, then extract the .zip to a folder
  • UEFI boot to Win10 ISO > CMD prompt. Use DISKPART to check OS disk number (it's typically 0), list volume (to find USB drive)
  • Open CMD and cd to the GDisk directory you made on the USB drive
  • Run GDisk against disk 0 like so: gdisk64.exe 0:
  • Type "Y" to start GDisk, then "?" to display commands
  • w = write table to disk and exit

Once this is done your partitions have been converted from MBR to GPT, but your system partition (boot loader) is now broken. Follow the steps here to recreate it: https://social.technet.microsoft.com/wiki/contents/articles/14286.converting-windows-bios-installation-to-uefi.aspx

Finally, change your BIOS boot mode to UEFI and you're done!

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