Overview

Older systems and OEM installs typically use MBR (Master Boot Record) partition tables with a Legacy BIOS boot. Modern systems require GPT (GUID Partition Table) and UEFI to enable Secure Boot, TPM 2.0, and Windows 11 upgrades. Microsoft ships a built-in tool — MBR2GPT.EXE — that converts the partition table in-place without data loss or reinstall.

Before you start — checklist

  • Back up your data. This process is non-destructive in normal conditions, but hardware surprises happen. Full image backup first.
  • Confirm your motherboard actually supports UEFI (nearly all boards from 2011+ do).
  • You need Windows 10 version 1703 or later. Check: winver in Run.
  • The disk must have 3 or fewer partitions on the MBR disk (MBR2GPT limitation). If you have more, consolidate first.
  • Run from an elevated command prompt (Run as Administrator).

Step 1 — Validate the disk

Open an elevated Command Prompt and run the validation pass first. This makes no changes — it just checks compatibility:

mbr2gpt /validate /disk:0 /allowFullOS

If it reports "Validation completed successfully", proceed. If it fails, read the log at C:Windowssetuperr.log and resolve any issues before continuing.

Note: /disk:0 is typically the system drive. Confirm your disk number in Disk Management (diskmgmt.msc) first.

Step 2 — Convert the disk

Still in the elevated Command Prompt:

mbr2gpt /convert /disk:0 /allowFullOS

The tool will:

  1. Shrink an existing partition slightly to create space for the EFI System Partition (ESP).
  2. Create a new EFI System Partition (ESP) and MSR (Microsoft Reserved Partition).
  3. Update the boot configuration data (BCD) for UEFI.
  4. Update the Windows Recovery Environment.

The disk is now GPT. Do not reboot yet — the BIOS is still set to Legacy.

Step 3 — Switch the firmware to UEFI

  1. Reboot and enter your BIOS/UEFI setup (Del, F2, or F10 — varies by board).
  2. Find the Boot Mode or CSM (Compatibility Support Module) setting.
  3. Change Legacy / CSM → UEFI only.
  4. In the boot order, make sure the Windows Boot Manager UEFI entry is first (it should appear automatically).
  5. Save and exit (F10).

Step 4 — Verify UEFI is active

Once Windows boots, open an elevated Command Prompt and run:

bcdedit | findstr /i "winpe"

Or check via System Information:

msinfo32

Look for BIOS Mode: UEFI. If it shows UEFI, the conversion is complete.

You can also confirm in Disk Management — the system disk should now show a EFI System Partition at the front.

Enable Secure Boot (optional but recommended)

With UEFI active you can now enable Secure Boot:

  1. Re-enter BIOS setup.
  2. Navigate to Security → Secure Boot.
  3. Set to Enabled.
  4. If prompted to clear Secure Boot keys and restore defaults, do so.
  5. Save and exit.

Troubleshooting

  • "Disk layout validation failed" — You likely have more than 3 MBR partitions. Use Disk Management to remove or merge recovery/utility partitions, then retry.
  • Windows fails to boot after BIOS switch — Re-enter BIOS, temporarily re-enable CSM, boot Windows, then check C:Windowssetuperr.log for MBR2GPT errors.
  • UEFI boot entry missing — Boot from Windows 10 install media, choose Repair, open Command Prompt, and run bcdboot C:Windows /s S: /f UEFI (replace S: with your EFI partition letter).
  • BitLocker-encrypted drive — Suspend BitLocker before converting: manage-bde -protectors -disable C:. Re-enable after verifying the UEFI boot.

References

  • Microsoft Docs: MBR2GPT — learn.microsoft.com/windows/deployment/mbr-to-gpt
  • Disk Management (diskmgmt.msc)
  • System Information (msinfo32)