You’re trying to convert your hard drive from MBR to GPT using MBR2GPT, dreaming of smooth Windows upgrades or booting into UEFI like a tech wizard… and boom! You get hit with that annoying validation error. Don’t worry—many have faced it before. And there are several ways to fix it without feeling like you’re fighting a tech dragon.
TL;DR (Too Long; Didn’t Read)
If you see the MBR2GPT validation failed error, it usually means your disk isn’t conversion-ready. Check if the drive is MBR, has enough partitions, and meets Windows requirements. Backup your data before making changes. Follow a few easy steps and you’ll be booting with GPT in no time.
What Is MBR2GPT?
Before diving into errors and fixes, here’s a super short explanation. MBR2GPT is a tool by Microsoft. It lets you convert a hard drive from MBR (Master Boot Record) to GPT (GUID Partition Table) without deleting data. Sounds cool, right?
You need GPT if you want to use UEFI instead of Legacy BIOS. It’s also required for newer Windows features and support.
Why Does the Validation Fail?
The MBR2GPT tool checks your disk before making the switch. If it fails the validation step, it’s usually due to one of these things:
- Your disk isn’t using MBR (already GPT).
- The disk has too many partitions.
- There’s not enough space in the system reserved partition.
- The disk isn’t being flagged as “system” or doesn’t contain the OS.
You can see these errors in the command prompt as messages like:
MBR2GPT: Validation failed. See logs for details.
Okay, enough talk. Let’s fix it!
Step 1: Open Command Prompt as Admin
Press Windows key + X and select Command Prompt (Admin) or Windows Terminal (Admin).
Now type:
mbr2gpt /validate /disk:0 /allowFullOS
This runs the validation on Disk 0 (your main drive). Change the disk number if needed.
Step 2: Check Your Disk Layout
Next, type this command to check your partitions:
diskpart
list disk
Check if it says MBR under the GPT column. If it’s already GPT, you don’t need MBR2GPT!
Now type:
select disk 0
list partition
If you see more than 3 partitions, that may be the problem. MBR2GPT supports only up to 3 primary partitions on MBR before converting.
Fix: You’ll need to remove or merge partitions if you have more than 3. Back up your data first!
Step 3: Check System Partition Size
The System Reserved Partition should have at least 15MB free space for MBR2GPT to work.
To check space, go back to Disk Management:
- Press Windows + R and type diskmgmt.msc.
- Right-click the System Reserved partition and click Properties.
If it’s too small, you’ll either have to expand the partition or delete and recreate it. Both steps can be tricky, so it’s best to use tools like MiniTool Partition Wizard.
Step 4: Make Sure You’re on the Right Volume
Your system partition—the one with Windows installed—must be marked as Active and be the system disk. If MBR2GPT doesn’t detect Windows on the selected disk, it will fail.
Check this with:
diskpart
select disk 0
list partition
Look for a partition that says System or Boot.
If your Windows is on another disk, you’ll need to adjust disk numbers in the MBR2GPT command.
Step 5: Clean Boot and Retry
Sometimes, the validation fails because the tool doesn’t have full access. Restart your PC into Windows Recovery Environment (WinRE) or use a bootable Windows Install USB drive.
On the blue menu:
- Choose Repair your computer.
- Go to Command Prompt.
Then run:
mbr2gpt /validate /disk:0
mbr2gpt /convert /disk:0
No /allowFullOS flag is needed here because you’re already out of the main OS.
Bonus Tip: Check and Fix the BCD Store
If your conversion completed but the system doesn’t boot, fix the BCD (Boot Configuration Data).
Boot into recovery again and run:
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcd
Still stuck? Run:
bcdboot C:\Windows
This recreates boot files and makes your system bootable again.
Pro Tips to Avoid MBR2GPT Headaches
- Always back up your data. Always. We mean it.
- Check your partition count before running the tool.
- Don’t run MBR2GPT on disks with obscure bootloaders (like Linux GRUB).
- Use Windows 10 v1703 or later. Earlier versions won’t support the tool.
- UEFI must be enabled in BIOS if you plan to boot after conversion.
Can I Use a Third-Party Tool Instead?
Yes! Tools like AOMEI Partition Assistant or EaseUS Partition Master can convert MBR to GPT with a click. They’re mostly drag-and-drop. But they might require a Pro version, and be warned—they’re not Microsoft certified. So sometimes they miss stuff—like making the disk bootable.
Summary: From Error to Victory!
MBR2GPT validation failed? It happens. But you’re now armed with the knowledge to tackle it like a PC pro.
Quick Checklist:
- Make sure the disk is MBR.
- Limit partitions to 3 or fewer.
- Ensure enough system reserved space.
- Boot into recovery for a cleaner run.
Done? High five! 🎉 Now you can enjoy the magical powers of GPT and UEFI without losing any data – just like that.
