How to fix MBR2GPT /validate failure, "cannot find OS partitions for disk 0"?

Question

How to fix MBR2GPT /validate failure, "cannot find OS partitions for disk 0"?

TACO3141 0

Hello,

I'm trying to convert my disks from their current MBR format to GPT format. I entered windowsRE command prompt as I heard was safest, then entered the command "mbr2gpt /validate". I was met with "cannot find OS partitions for disk 0" as a failure message. I exited command prompt and restarted to search for the fix for this, when i was met with a BSOD with error code INACCESSIBLE_BOOT_DEVICE. I entered windows RE by hard restarting 3 times, and Start-Up Repair fixed it automatically for me.

User's image

This is my disk setup. Let me know if any more info is needed.

0 comments

2 answers

Your answer

Answer 1

Volume Z 1,650 Volunteer Moderator

Taco,

while this is not the main problem, you should give up that Recovery Partition before converting to GPT.

A new (EFI) System Partition will be created by either MBR2GPT or yourself, and this one should be bordering the right edge of Disk 0. The recovery partition is basically in the way of this process.

Also 508 MB is not up to the current standard of keeping Windows RE updated. It should be 800 MB at minimum.

Suggest starting by abandoning the Recovery Partition. Windows RE needs to be disabled before. Please run

reagentc /info

and post the output.

TACO3141 0 Reputation points

2026-08-22T01:16:20.97+00:00

@Emma1-N Just tagging you so you see this comment too.
Volume Z 1,650 Reputation points Volunteer Moderator

2026-08-22T04:32:18.7433333+00:00
Please run

reagentc /disable diskpart select disk 0 select partition 2 delete partition override select partition 1 extend

and post a new image of disk management.
TACO3141 0 Reputation points

2026-08-22T18:07:32.6933333+00:00
Volume Z 1,650 Reputation points Volunteer Moderator

2026-08-22T18:53:37.1866667+00:00

We will now create a new System and future EFI System Partition after drive C. The partition must be at least 100 MB to qualify for EFI, i.e. twice the size of the current System Reserved.

Please shrink drive C by 100 MB in disk management. In the unallocated space, create an NTFS formatted partition with a drive letter. Post a new image of disk management.
TACO3141 0 Reputation points

2026-08-22T21:55:12.2933333+00:00

Did my best here. Not too sure how to turn it into a system partition. Wizard included so you have the settings.
Volume Z 1,650 Reputation points Volunteer Moderator

2026-08-22T22:41:00.0133333+00:00

All good so far, except the partition has turned out 1 MB too small which may defy our purpose. Shrink drive C by another 2 MB and create a new System partition next to it.

In case of doubt it's better to have the partition 101 MB than 99.
TACO3141 0 Reputation points

2026-08-22T23:52:41.6366667+00:00

It wouldn't create a new partition for 2 MB for whatever reason, so I just added 100 more MB.
Volume Z 1,650 Reputation points Volunteer Moderator

2026-08-23T00:11:28.34+00:00
OK, but you need to assign a drive letter. Suggest using D again. Assign drive letter D in disk management. Then:

Mark drive D as active in disk management.

Run
bcdboot C:\Windows /s D: /f ALL
Post the output of cmd as well as disk management.
TACO3141 0 Reputation points

2026-08-23T01:34:46.9366667+00:00
Volume Z 1,650 Reputation points Volunteer Moderator

2026-08-23T01:44:40.5833333+00:00
Very good. Let's retire System Reserved. Run

diskpart select disk 1 select partition 1 inactive

and reboot. Post a new image of disk management.

Answer 2

Emma1-N 10,755 Microsoft External Staff Moderator

Hello TACO3141,

Thank you for providing the screenshot and additional details.

Based on your disk layout, the most likely reason MBR2GPT reports "Cannot find OS partition(s) for disk 0" is that Windows is installed on Disk 0, while the System Reserved boot partition is located on Disk 1. In this configuration, MBR2GPT may be unable to identify all required boot components on the disk being converted.

Since Startup Repair successfully resolved the INACCESSIBLE_BOOT_DEVICE error, I would not recommend running MBR2GPT again until the current boot configuration is verified.

Please run the following commands from an elevated Command Prompt in Windows and share the results: bcdedit

Then:

diskpart
list disk
list volume
exit

The output will help determine whether the boot files are stored on Disk 1 and whether they need to be moved before attempting the conversion.

Looking for your update.

TACO3141 0 Reputation points

2026-08-14T18:26:28.63+00:00
Emma1-N 10,755 Reputation points Microsoft External Staff Moderator

2026-08-15T10:04:07.0166667+00:00

Hello TACO3141,

Thank you for providing the results. They confirm that Windows is installed on Disk 0 (C:), while the System Reserved partition containing the boot files is on Disk 1. This split configuration is the most likely reason why MBR2GPT cannot find OS partition(s) for disk 0.

Before proceeding further, please run: mbr2gpt /validate /disk:0 /allowFullOS

and share the complete output. That will show whether the split boot configuration is the only issue or if there are additional partition layout problems preventing the conversion.
TACO3141 0 Reputation points

2026-08-15T18:14:20.65+00:00

Should I run this in WindowsRE command prompt or a regular administrator command prompt? Sorry, I just want to be sure.
Emma1-N 10,755 Reputation points Microsoft External Staff Moderator

2026-08-17T03:17:32.2133333+00:00
Hello TACO3141,

Please run the command from a regular Administrator Command Prompt in Windows, not from Windows RE.

Start Windows normally.

Right-click Start and select "Command Prompt (Admin)" or "Windows Terminal (Admin)".

Run: mbr2gpt /validate /disk:0 /allowFullOS

The /allowFullOS switch is specifically intended for running MBR2GPT from the full Windows environment. Once it completes, please post the full output so we can see exactly what validation check is failing.
Emma1-N 10,755 Reputation points Microsoft External Staff Moderator

2026-08-18T03:51:10.4566667+00:00

Hello TACO3141,

Is there any update from you? Please let me know, I will be here to assist you.
TACO3141 0 Reputation points

2026-08-18T18:48:13.9+00:00

Sorry about the late reply.
Emma1-N 10,755 Reputation points Microsoft External Staff Moderator

2026-08-21T03:54:14.32+00:00

Hello TACO3141,

I am sorry for the lately response. Please you can check the Volume Z's answer above. After that, let me know the result, I will continue to assist.