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💽 Disk Conversion

Convert MBR to GPT with MBR2GPT Complete Guide

Your SSD is stuck on the old MBR partition style, and Windows needs GPT for UEFI booting. Microsoft's own MBR2GPT tool converts the system disk in place — no reinstall, no formatting — when the disk passes its checks.

✍️ Technical Anshu🪟 Windows 10 / 11⏱️ 8 minute read
🎥 Watch the full video — the live conversion, from the first check to the firmware switch at the end

A lot of SSDs end up stuck in MBR because Windows was once installed in Legacy BIOS mode, or the drive was cloned from an older machine. The fix is not a reinstall. Windows 10 (version 1703 and later) and Windows 11 carry a built-in tool called MBR2GPT that rewrites the partition table of the system disk to GPT while your partitions, files and Windows installation stay where they are. This guide walks the whole job in the safe order: confirm, back up, validate, convert, switch the firmware — because the command alone is only half the job.

Administrator: Command Prompt — validating disk 0 (example output)
C:\Windows\system32> mbr2gpt /validate /disk:0 /allowFullOS MBR2GPT: Attempting to validate disk 0 MBR2GPT: Retrieving layout of disk MBR2GPT: Validating layout, disk sector size is: 512 bytes MBR2GPT: Validation completed successfully

👆 A clean validation looks like this — example output. Nothing on the disk has changed yet at this stage

1

First, Confirm Your Disk Really Is MBR

Do not convert a disk that is already GPT. Two quick checks:

  1. Press Windows + R, type diskmgmt.msc and press Enter. Right-click the grey Disk 0 label on the left (not a partition) → Properties → Volumes tab. The Partition style line says Master Boot Record (MBR) or GUID Partition Table (GPT).
  2. Also run msinfo32 and read the BIOS Mode line. Legacy means your PC currently boots the old way, and this conversion applies to you. If it already says UEFI, your system disk is almost certainly GPT already.

One more condition, and it is not optional: your PC's firmware must support UEFI. Microsoft states this plainly — make sure the device supports UEFI before attempting to convert. A Legacy-only motherboard cannot boot a GPT system disk, and no command changes that.

2

Back Up Before You Touch the Partition Table

MBR2GPT is designed to keep your data, and on a healthy disk it normally does. It still rewrites the partition table of the disk your operating system lives on. A power cut, a failing SSD or a wrong disk number at the wrong second is exactly how careful people lose data.

  1. Copy important files to another disk or external drive, or take a full disk image.
  2. If BitLocker is turned on for the Windows drive, suspend it before converting — Microsoft's own guidance for this tool.
  3. Keep a bootable Windows USB nearby, just in case.

Ten minutes of backup is what makes the rest of this guide a relaxed job instead of a gamble.

3

Know the Rules Before You Run Anything

MBR2GPT validates the disk against a fixed rule list from Microsoft, and it refuses if even one rule fails:

  1. The disk is currently MBR.
  2. There is enough space not occupied by partitions to store both GPTs — 16 KB + 2 sectors at the front of the disk and 16 KB + 1 sector at the end.
  3. There are at most three primary partitions in the MBR table.
  4. One partition is set as active and is the system partition.
  5. The disk has no extended or logical partitions.
  6. The BCD store on the system partition contains a default OS entry pointing to an OS partition.
  7. The volume ID can be retrieved for every volume that has a drive letter.
  8. Every partition is of an MBR type Windows recognises — or one you map yourself with the /map option.

The BCD rule is the one that tells you what this tool really is: it converts the Windows system disk. A plain data disk has no BCD, so it fails this check by design — that case has its own page in this series.

4

Step 1 — Find the Disk Number

Open Command Prompt as administrator and run:

  1. diskpart
  2. list disk
  3. exit

Note the number of your Windows disk — usually Disk 0. Check it by size, and look at the Gpt column: a star there means the disk is already GPT. If you skip /disk: in the next steps, MBR2GPT uses the system disk automatically — but on a multi-disk PC, naming the disk yourself is the safer habit.

5

Step 2 — Validate. Always Validate First

Validation changes nothing on the disk. It simply runs the rule list above and tells you the verdict:

  1. In the same admin Command Prompt, run mbr2gpt /validate /disk:0 /allowFullOS (replace 0 with your disk number).
  2. Validation completed successfully means the disk is eligible. Convert whenever you are ready.
  3. Disk layout validation failed means stop. The tool writes its reasoning to setupact.log and setuperr.log in the Windows folder — read the failure like a checklist item, fix that one thing, and validate again.

The /allowFullOS flag matters here. MBR2GPT was designed to run from the Windows Preinstallation Environment (WinPE); this flag is what permits it to run from your normal, fully booted Windows. The checks are identical either way.

6

Step 3 — Convert with One Command

When validation passes, the conversion itself is a single command:

  1. Run mbr2gpt /convert /disk:0 /allowFullOS
  2. The tool re-validates, creates the EFI System Partition and the Microsoft Reserved partition, rewrites the table as GPT, and rebuilds the boot configuration for UEFI booting. If there is no spare room, it attempts to shrink the system partition slightly to make space for the EFI partition.
  3. It normally finishes in about a minute. Leave the PC alone while it works — do not restart it mid-run, whatever the screen seems to be doing.

Full syntax, for reference: MBR2GPT /validate|convert [/disk:<diskNumber>] [/logs:<logDirectory>] [/map:<source>=<destination>] [/allowFullOS]. Running the same commands from a WinPE or recovery Command Prompt works too — there the /allowFullOS flag is simply not needed.

7

Step 4 — Switch the Firmware to UEFI, or It Will Not Boot

This is the step people skip, and it is the step the tool itself warns you about. Your disk is now GPT, and a GPT system disk only boots in UEFI mode. If you simply reboot, the PC will still try Legacy booting and will report that no bootable device was found. Nothing is lost — the firmware is just knocking on the wrong door.

  1. Restart and enter firmware setup (usually Del or F2 during start-up), or use Windows Settings → Recovery → Advanced startup → UEFI Firmware Settings.
  2. Find the boot mode setting and change it from Legacy / CSM to UEFI.
  3. Save and exit. Windows should now start normally.
8

Step 5 — Verify the Result

  1. Run msinfo32 — BIOS Mode should now read UEFI.
  2. Open Disk Management → your disk's Properties → Volumes — Partition style should read GUID Partition Table (GPT).
  3. If BitLocker was suspended, resume it now.

That is the complete job: same Windows, same files, same programs — booting the modern way, on a partition style that has room to grow.

🔴 Honest line: "One command" is true — but the full job is backup → validate → convert → firmware switch. Skipping the backup or the firmware step is how a one-minute conversion turns into an evening of recovery work.

🎥 Watch the Complete Conversion in the Video

The video runs the complete conversion live — checks, the command itself, and exactly what to change in the BIOS screen afterwards.

▶️ Watch Full Video
❓ Frequently Asked Questions

Does MBR2GPT delete my files or programs?

No. The tool is built to convert the partition style in place: partitions, files and the Windows installation stay where they are. It only rewrites the partition table and boot configuration. A backup is still essential, because the tool is writing to the most important structures on the disk.

Why does it say MBR2GPT can only be used from the Windows Preinstallation Environment?

That is the tool's default design — it expects to run from WinPE. Adding /allowFullOS to the command permits the exact same validation and conversion from your normal Windows desktop, which is how most people run it.

My PC booted to a black screen after conversion. Is my data gone?

Almost certainly not. This is the classic missed step: the firmware is still set to Legacy/CSM, and a GPT system disk cannot boot that way. Enter the firmware setup, switch the boot mode to UEFI, save, and Windows will start with everything intact.

This guide is based on the Technical Anshu MBR to GPT conversion video • Stuck on a step? Ask in the video comments — you will get a reply 🙏