Convert your disk from MBR to GPT for Secure Boot

Steps

1. 01

   Confirm you actually need this, and that the PC can take it

   • Win+Rmsinfo32›System Summary. You only need this if BIOS Mode reads 'Legacy'; if it already says UEFI there is nothing to convert
   • Confirm the current partition style: open PowerShell and run Get-Disk | ft -Auto, then read the Partition Style column — it says MBR or GPT
   • Check the machine supports UEFI at all before going any further

   ✓You should see: BIOS Mode: Legacy in msinfo32 and Partition Style: MBR in Get-Disk — that pair is what this fix is for.

   !Careful: Microsoft is explicit that you must make sure the device supports UEFI before attempting the conversion. Converting a disk on a PC whose firmware cannot boot UEFI leaves you with a machine that will not start.

2. 02

   Back up, properly, before anything else

   • Take a full backup to a separate drive and confirm you can open a file from it
   • Make sure you can reinstall Windows if you have to: have your sign-in details and any licence keys to hand

   !Careful: The tool itself prints 'These changes cannot be undone!' before it runs. It does not modify or delete your data by design, but a conversion that fails partway leaves the disk in a state you recover from with a backup, not with another command.

   Also see: Back up properly (3-2-1 rule)

3. 03

   Suspend BitLocker if the drive is encrypted

   • Search Start for 'Manage BitLocker' and check whether the system drive is encrypted
   • If it is, suspend protection before converting — mbr2gpt refuses to convert a disk with an encrypted volume and returns error code 6
   • After the conversion, Microsoft's documented step is to delete the existing protectors and recreate them to resume BitLocker

   !Careful: Have your BitLocker recovery key in front of you before you touch anything. Firmware changes are exactly the kind of event that makes Windows ask for it on the next start, and Microsoft cannot retrieve or recreate a lost key.

   Also see: Turn on full-disk encryption (BitLocker)

4. 04

   Validate first, then convert

   • Open an elevated Command Prompt: type 'command prompt' in the taskbar search box, right-click Command Prompt and choose Run as administrator
   • Get your disk number first: run Get-Disk in PowerShell and read the Number column for the disk Windows is on — that number is the N in /disk:N below, and it is not always 0
   • Validate without changing anything: mbr2gpt /validate /disk:N /allowFullOS
   • Only if validation completes successfully, convert: mbr2gpt /convert /disk:N /allowFullOS
   • If anything fails, the detail is in setuperr.log and setupact.log in %windir%

   ✓You should see: 'MBR2GPT: Conversion completed successfully', followed by the tool's own reminder that you must switch the firmware to UEFI mode before the system can boot properly.

   !Careful: Do not skip /validate. Microsoft's tool checks the disk is MBR, is the system disk, has at most three primary partitions, no extended or logical partitions, a valid boot entry and room for the GPT headers. If any check fails the conversion does not proceed — that refusal is protecting you.

   Why?

   /allowFullOS lets the tool run inside Windows rather than from a recovery environment. Microsoft notes that in this case the existing system partition is in use, so a new EFI system partition is created by shrinking the Windows partition instead.

5. 05

   Switch the firmware to UEFI, then verify

   • Restart into firmware settings and change the boot mode from Legacy/CSM to UEFI
   • The setting is usually called Boot Mode, CSM or Legacy Support — switching CSM off is what selects UEFI on most boards
   • The PC will not start until you do — this is the documented final step, not a sign of failure
   • While you are there, enable Secure Boot if that was the goal
   • Back in Windows, confirm with Get-Disk | ft -Auto that Partition Style now reads GPT, and with msinfo32 that BIOS Mode is UEFI and Secure Boot State is On
   • Restart once more and check it still starts cleanly before you consider the job done

   ✓You should see: Partition Style GPT, BIOS Mode UEFI, Secure Boot State On — and two clean restarts.

   Why?

   If the old MBR system partition was not reused, it may now show up with a drive letter you don't want. Microsoft's guidance is to hide that letter manually rather than delete the partition.