Error when using MBR2GPT Tool

Question

Error when using MBR2GPT Tool

Craig Abrahams 0

Good morning all

Having an issue when using the aforementioned tool.

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Disk Manager shows the following

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I do have an intermittent issue with the PC failing to find an operating system after POST where I have to restart, go into the BIOS and then manually select volume D (Disk 0) above. This happened after upgrading from the original HDD to NVME which I assigned to being the C drive and believed I had migrated all the windows files to.

Looking to move to GPT as I want to finally upgrade to windows 11 and for that, I need Secure Boot.

I am not an advanced user by any means so really appreciate any support.

Cheers

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Answer 1

Volume Z 1,485 Volunteer Moderator

It does not matter what changes you make to disk 1 as long as you are booting off disk 0.

You should try a totally different approach including using MBR2GPT from outside Windows.

Step 1: Move the "Active" flag from drive C to drive G. Mark drive G as active in disk management.

Step 2: Add boot files to drive G. Run

bcdboot C:\Windows /s G: /f ALL

Now you need to get off booting from disk 0. Preferably remove the disk.

Run

reagentc /boottore

and reboot to recovery environment. Inside recovery environment run command prompt and try to convert disk 1 without the AllowFullOS switch so System reserved gets converted to EFI.

Craig Abrahams 0 Reputation points

2026-06-01T13:19:14.73+00:00

Firstly, thank you Volume Z for taking the time to respond.

So, I believe I have followed the first part of your guide correctly (up to and included the "remove the disk" part. I have rebooted and it worked great not having to point BIOS towards the D drive (disk 0)

Looking at disk management, I now have the following.

The next section where I run reagentc /boottore, assume that is in command prompt as well, will that force a reboot into recovery or do I need to do that myself. If its the latter, how do I get into a recovery environment, is that the same as Safe Mode?

If that is the case, assume I then use the following command mbr2gpt.exe /convert /disk:0

I am assuming its disk 0 as that is where my active OS loading drive is now?

Again, thank for your help, its great to learn from those who have done this before, or at least know what they are doing.
Volume Z 1,485 Reputation points Volunteer Moderator

2026-06-01T15:40:12.9933333+00:00

Looking good so far. Yes, boottore will force a boot into recovery environment. Yes, the former disk 1 is now disk 0. Please run the above command. Post a new screenshot after the conversion as I suggest making some improvements to the recovery partition.
Craig Abrahams 0 Reputation points

2026-06-01T15:50:37.66+00:00

Thanks again.

I tried the first command and got the following

The second command is what came up as a potential solution but also, sadly didn't resolve.

Is there another option I could look at?
Volume Z 1,485 Reputation points Volunteer Moderator

2026-06-01T15:57:15.6433333+00:00

That's bad, you don't have Windows RE available. This is going to be fixed by a feature upgrade, but for now it requires you to create installation media or run MBR2GPT inside Windows, which would create the EFI System Partition in an unfavorable spot while leaving the sizeable System Reserved partition unused. :/
Craig Abrahams 0 Reputation points

2026-06-01T16:32:21.0366667+00:00

So, is it a case of running MBR2GPT now on drive 0 and take the hit on losing efficiency on drive space or flattening the drive and going with a fresh install and doing a lot of reinstalling of programs/files etc?
Volume Z 1,485 Reputation points Volunteer Moderator

2026-06-01T16:35:20.8+00:00

No, you would use the "Repair" capacity of install media to still use command prompt and run MBR2GPT from there.

Answer 2

DaveM121 (2) 14,210 Volunteer Moderator

The mbr2gpt command can only be run on the disk containing the Windows operating system, in your screenshot, that seems to be Disk 1, not Disk 0, have you tried to run that command on Disk 1?

Craig Abrahams 0 Reputation points

2026-05-31T11:14:49.3433333+00:00

Hey Dave, thanks for commenting

Sadly not, I tried the AI answer which was to use the following command

mbr2gpt.exe /convert /disk:1 /allowFullOS

It gave this as a result

It recommended to look in Setuperr file but that shows the following

I have also, tried the same command but with drives 0 and 2 just to cover all bases but alas, it still doesnt fix it.
DaveM121 (2) 14,210 Reputation points Volunteer Moderator

2026-05-31T11:34:14.24+00:00

Try running this command first, when that completes, try running the mbr2gpt command again.

bcdboot C:\Windows /s C:

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Error when using MBR2GPT Tool

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