Mbr2gpt conversion problems

Question

Preparing for win 11 pro install. Followed MS tech bulletin on process to use mbr2gpt tool to change boot drive in preparation to change from Legacy Boot to UEFI boot. Conversion seemed successful. REceived ‘Unable to update Boot Configuration Data’. Error with ‘Unable to update BCD’. Now locked in Windows RE. Can see disks and volumes within the RE command prompt. Boot drive was previously Disk 0 (C:). Windows installation now in Disk 2 (E:). DISKPART shows my drive 0-3 are GPT. Followed MS tech bulletin for rebuild BCD. Bootrec /rebuildbcd identifies Windows installation on E: when answering Y to add installation to boot list I receive ‘requested system device cannot be found’. Where do I go now?

Answer 1

Hello Brian Flynn,

Welcome to Microsoft Q&A forum.

Before we proceed, may I have a few photos about the situation to check for you? Please remember to remove all personal information before posting.

I'm waiting for your reply.

Answer 2

After rebuilding the boot files on the EFI boot partition, now Z, and pointing to windows on E. BIOS settings changed to UEFI. Reboots to a blue screen with ‘inaccessible boot device’ error

Answer 3

What looks like the 100MB FAT32 EFI system partition shows as Disk | Volume 3 with no drive letter in previous list volume command.. Most recent list disk command doesn’t show disk |volume 3 but shows in list volume command. Sel disk command returns no disk | volume 3.

Answer 4

MBR2GPT completed the disk conversion but failed when updating the Boot Configuration Data (BCD), so the firmware and BCD no longer point to a valid boot entry. The disk and partitions are already GPT; the next steps are to ensure the correct EFI System Partition (ESP) exists and then repair boot files/BCD from Windows RE.

A practical recovery path based on the documented behavior of MBR2GPT and BCD:

1. Confirm disk layout and GPT status
   1. In Windows RE, open Command Prompt.
   2. Run diskpart.
   3. Run list disk and confirm the system disk (the one that contains the Windows volume now seen as E:) shows an asterisk in the Gpt column (as expected after MBR2GPT).
   4. Run sel disk <n> for that disk and then list vol to identify:
      • The Windows volume (currently E: in RE).
      • The EFI System Partition (usually 100–300 MB, FAT32). Assign it a drive letter if it doesn’t have one, e.g.:

                sel vol <efi-volume-number>
                assign letter=Z
        

   5. Exit DiskPart with exit.
2. Rebuild boot files on the EFI System Partition With the EFI partition mounted as Z: and Windows on E:, run:

      bcdboot E:\Windows /s Z: /f UEFI
   

   This copies the boot files from the Windows installation to the EFI System Partition and recreates the BCD store for UEFI.
3. Check firmware boot mode and order After bcdboot succeeds:
   1. Reboot and enter firmware/BIOS setup.
   2. Ensure the system is set to UEFI Only (or equivalent) and that any Compatibility Support Module (CSM) is disabled.
   3. Make sure the Windows Boot Manager entry on the converted disk is first in the boot order.
4. Verify boot and Secure Boot (optional) Once Windows boots:
   1. Run msinfo32 and confirm:
      • BIOS Mode: UEFI
      • Secure Boot State: On (if Secure Boot is enabled in firmware)

If bcdboot still fails, review the MBR2GPT logs (setupact.log, setuperr.log) in %windir% from Windows RE to identify any disk-layout or ESP issues, as documented in the troubleshooting section.

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References:

• MBR2GPT.EXE
• MBR2GPT tool test guidance
• Windows Setup: Installing using the MBR or GPT partition style