November 11, 2026

Windows 10 MBR2GPT Validation Failed: Why the Disk Conversion Tool Refuses to Run

You are preparing to switch your PC from Legacy BIOS to UEFI, perhaps to enable Secure Boot or meet upgrade requirements. You run the mbr2gpt tool, and it stops with “Disk layout validation failed for disk 0” or “Cannot find OS partition(s) for disk 0.” When Windows 10 MBR2GPT validation failed appears, the tool is protecting you from a conversion it cannot do safely.

MBR2GPT converts a disk from the older MBR partition style to GPT without deleting data. To do this, it needs room to create a small EFI system partition, and the disk must have a simple layout it understands.

Validation fails when the disk has too many partitions, when there is no slot free space for the new partition, when the Windows partition cannot be identified, or when the tool is run without the right options.

Reading the log tells you exactly which rule failed.

Possible Causes

Validation fails for these reasons:

• The disk has more than three primary partitions, the limit MBR2GPT accepts.
• There is not enough unallocated space for the new EFI partition.
• The tool was run from Windows without the /allowFullOS switch.
• Extended or logical partitions, or a second operating system, are present.

BitLocker can also block conversion until protection is suspended.

Quick Fixes to Try First

Run the tool correctly and read the log.

1. Open Command Prompt as administrator.
2. Type mbr2gpt /validate /disk:0 /allowFullOS and press Enter, adjusting the disk number if needed.
3. Open C:\Windows\setupact.log and scroll to the end for the reason it failed.
4. Open Disk Management to count partitions on the disk.
5. Suspend BitLocker from Control Panel > BitLocker Drive Encryption if it is on.

Advanced Steps for Stubborn Cases

Fix the layout carefully.

1. If there are four partitions, back up and remove an unused one, such as an old OEM tools partition, only if you are sure it is not needed.
2. Shrink the Windows partition by about 1 GB in Disk Management to leave free space.
3. Convert logical partitions or move data off extended partitions before trying again.
4. Run the validation from the Windows Recovery Environment Command Prompt without /allowFullOS as an alternative.
5. Once validation passes, run mbr2gpt /convert, then switch the firmware to UEFI mode.

If the recovery partition is the issue, the log will mention it, and you can recreate recovery later with reagentc.

Before You Go Further: Keep Your Files Safe

Make a full backup or system image before changing partitions. Deleting the wrong partition can make Windows unbootable, so check each one’s size and label first.

Conclusion

MBR2GPT validation fails when the disk layout does not meet its rules. Use /allowFullOS, read setupact.log, reduce partitions, free some space, and convert only after a backup.