If your PC boots Windows 10 in Legacy BIOS (CSM) mode on an MBR disk, you can switch to UEFI without reinstalling Windows. Microsoft’s built-in MBR2GPT tool converts the system disk from MBR to GPT in place, then you change the firmware setting to UEFI. UEFI is required for Secure Boot and for upgrading to Windows 11. This guide covers checking your current mode, preparing, converting safely and fixing boot problems.
Quick Answer
Back up your PC and suspend BitLocker. Open Command Prompt as administrator and run mbr2gpt /validate /disk:0 /allowFullOS. If validation succeeds, run mbr2gpt /convert /disk:0 /allowFullOS. Restart into your firmware (BIOS) settings, switch Boot Mode from Legacy/CSM to UEFI, save and boot Windows.
Step 1: Check Your Current Boot Mode
- Press Windows + R, type msinfo32 and press Enter.
- Look at BIOS Mode. If it says Legacy, continue. If it says UEFI, you’re already done.
- Check the disk style: right-click Start > Disk Management, right-click Disk 0 > Properties > Volumes. Partition style shows MBR or GPT.
Step 2: Check Requirements
- Windows 10 version 1703 or later (run winver).
- A 64-bit version of Windows and a PC whose firmware supports UEFI (most PCs from about 2012 onward).
- The system disk has no more than three primary partitions and no extended/logical partitions (MBR2GPT needs room to add an EFI partition).
- BitLocker suspended: Control Panel > BitLocker Drive Encryption > Suspend protection.
- A full backup or system image, just in case.
Step 3: Validate the Disk
- Right-click Start and open Terminal (Admin) or Command Prompt (Admin).
- Run mbr2gpt /validate /disk:0 /allowFullOS (replace 0 if Windows is on another disk; check Disk Management).
- Look for Validation completed successfully. If it fails, see Troubleshooting below.
Step 4: Convert to GPT
- Run mbr2gpt /convert /disk:0 /allowFullOS.
- Wait for Conversion completed successfully. The tool creates an EFI system partition and installs UEFI boot files.
You can also run MBR2GPT from the recovery environment (Troubleshoot > Advanced options > Command Prompt) without /allowFullOS.
Step 5: Switch the Firmware to UEFI
- Restart and press the firmware key during startup (often F2, F10, Del or Esc), or use Settings > Update & Security > Recovery > Advanced startup > Restart now > Troubleshoot > Advanced options > UEFI Firmware Settings (only available once booted in UEFI; use the key otherwise).
- Find Boot Mode, CSM or Legacy Support and set it to UEFI (disable CSM).
- Optionally enable Secure Boot.
- Save and exit. Windows should boot normally.
Step 6: Confirm and Finish
- Run msinfo32 again; BIOS Mode should show UEFI, and Secure Boot State shows On if you enabled it.
- Resume BitLocker protection.
- Check that the recovery environment works: run reagentc /info and, if it shows Disabled, reagentc /enable.
Troubleshooting
- “Disk layout validation failed”: you may have four partitions or an extended partition. Remove unneeded partitions (such as an old recovery or data partition, after backing up) and try again.
- “Cannot find OS partition(s)”: check the disk number, and make sure you’re converting the Windows disk.
- PC won’t boot after conversion: you probably haven’t switched the firmware to UEFI yet, or the firmware is still set to CSM. Switch it, or switch back to Legacy temporarily to boot.
- No boot device after switching: make sure Windows Boot Manager is first in the UEFI boot order.
Frequently Asked Questions
Will I lose my files?
MBR2GPT converts in place without deleting data, but always back up first in case of power loss or errors.
Do I need this for Windows 11?
Yes. Windows 11 requires UEFI with Secure Boot capability. See our guide to turning on Secure Boot.
Is Windows 10 still supported?
Windows 10 reached end of support on October 14, 2025. Converting to UEFI is often the first step toward upgrading.
Summary
- Confirm Legacy mode in msinfo32 and back up.
- Suspend BitLocker and run mbr2gpt /validate.
- Run mbr2gpt /convert.
- Switch firmware to UEFI and optionally enable Secure Boot.
Kermit Matthews is a freelance writer based in Philadelphia, Pennsylvania with more than a decade of experience writing technology guides. He has a Bachelor’s and Master’s degree in Computer Science and has spent much of his professional career in IT management.
He specializes in writing content about iPhones, Android devices, Microsoft Office, and many other popular applications and devices.