How to Convert MBR to GPT Without Losing Data
MBR, GPT, Legacy and UEFI explained in plain words, then how to convert your Windows drive to GPT with Microsoft's own tool and switch to UEFI without losing files.
Key takeaway
Use Windows’ own MBR2GPT tool, then switch your BIOS to UEFI. No reinstall, no lost files.
In this article9 sections · 8 min read
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If you turned on Secure Boot for Windows 11 and your PC keeps going back to the BIOS instead of starting Windows, your system drive is almost certainly using the old MBR partition style. Secure Boot needs UEFI, and UEFI needs a GPT drive.
Here is how I converted my drive from MBR to GPT with Windows' own MBR2GPT tool, without losing a single file. I have used this on three PCs. I also explain what all these letters mean, because once you understand them, the steps make a lot more sense.
MBR, GPT, Legacy and UEFI in plain words
Every drive has a small map at the start that tells the computer how the drive is divided and where to find the operating system. There are two styles of map:
- MBR (Master Boot Record) is the old style from the 1980s. It works, but it has limits, such as a 2 TB maximum size.
- GPT (GUID Partition Table) is the modern style. It supports huge drives and keeps a backup copy of the map.
Your motherboard also has two ways of starting a PC:
- Legacy, also called CSM, is the old way. It can only start MBR drives.
- UEFI is the modern way. It starts GPT drives, and it is the only mode where Secure Boot works.
So the chain is simple: Windows 11 wants Secure Boot, Secure Boot needs UEFI, and UEFI needs GPT. If your drive is MBR, you convert it to GPT, then switch the BIOS from Legacy to UEFI.
Why you are here
Windows 11 needs a chain of four things
- No reinstall
- No lost files
- Built into Windows
Tip: Back up anything important first. MBR2GPT is built into Windows and safe, but it changes how your drive starts up, so a backup is cheap insurance.
What you need
- Windows 10 (version 1703 or newer) or Windows 11.
- An administrator account.
- Access to your BIOS.
- No more than three partitions on the Windows drive. MBR2GPT needs room to add one small startup partition.
- If you use BitLocker, suspend it first in Control Panel, then BitLocker Drive Encryption, then Suspend protection.
Step 1: Check that your drive is MBR
- Right-click Start and open Disk Management.
- Right-click the disk Windows is on (usually Disk 0, not a partition) and choose Properties.
- Open the Volumes tab. If Partition style says Master Boot Record (MBR), carry on.
You can also check how the PC is starting. Press Windows + R, type msinfo32 and press Enter. Next to BIOS Mode, Legacy means you are on the old system, and UEFI means you are already on the new one.
Step 2: Convert the drive to GPT
- Search for cmd, right-click Command Prompt and choose Run as administrator.
- Check the disk first: type mbr2gpt /validate /allowFullOS and press Enter. This only checks the drive and changes nothing. If you see "Validation completed successfully", you are good to go.
- Run the conversion: mbr2gpt /convert /allowFullOS
- Wait a minute or two. When you see "Conversion completed successfully", the drive is GPT and your files are untouched.
Two commands, two jobs
Validate first, convert second
The /allowFullOS part simply allows the tool to run from inside Windows. Without it, the tool expects to be run from the Windows recovery environment.
Windows will warn you that the firmware has to be switched to UEFI before the PC can boot properly. Do the next step before you try to start Windows again.
If validation fails
- Disk layout validation failed usually means there are too many partitions. Open Disk Management and look for extra partitions, such as an old recovery partition from the PC maker. Only remove a partition if you are sure you do not need it.
- Cannot find OS partition can happen when the drive was cloned or has an unusual layout. Running the tool from the recovery environment sometimes helps.
- Run the validate command again after each change. Only run the convert command when validation succeeds.
Step 3: Switch your BIOS to UEFI
Restart and keep pressing Delete (F2 on some boards) to open the BIOS. Menu names vary by brand. On my board it looked like this:
- Open Boot, then CSM (Compatibility Support Module).
- Set Boot Device Control to UEFI Only.
- Set Boot from Storage Devices to UEFI driver first. This one matters most: if storage is not on UEFI, Windows will not start.
- Leave Boot from Network Devices and PCI-E Expansion Devices on Legacy only.
- Press F10 to save and restart.
Step 3 · the BIOS path
Switch from Legacy to UEFI
- Delete at startup
- Boot
- CSM
- UEFI Only
- F10
On some boards it is one option: set CSM to Disabled.
On some boards the simplest version is a single option: set CSM to Disabled. That switches the whole board to UEFI.
Windows should now start normally, and Disk Management will show GUID Partition Table (GPT). You can now turn on Secure Boot. If you suspended BitLocker, resume it.
If you get a blue screen
It usually means the PC cannot find the drive to boot from yet. Turn the PC off for a few minutes, go back into the BIOS, choose Load defaults, restart, then set UEFI mode again as above. Your files are still there. It is a boot setting, not a lost drive.
Why I only use Microsoft's tool now
Before I found MBR2GPT, I tried a third-party partition tool for the same job, and it broke my Windows install. I wrote about that in what happened when a third-party tool cost me my Windows install. MBR2GPT is built into Windows, it checks the drive before changing anything, and it has worked every time I have used it.
The short version
Check that your drive is MBR, run mbr2gpt /validate, then mbr2gpt /convert, and switch the BIOS from Legacy to UEFI before you start Windows again. After that, Secure Boot and Windows 11 are within reach, and your files stay where they are.
