Cannot find room for the EFI system partition. MBR2GPT: Conversion failed

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Cannot find room for the EFI system partition. MBR2GPT: Conversion failed

Spencer Pack 30

I'm trying to upgrade my unlicensed (not really, I'll explain later) Windows 10 Pro to a licensed Windows 11 Pro. My system is, generally speaking, around 14 years old. I started with Windows XP on a clean install 14 years ago, and since then, I have upgraded my OS a few times. I have also migrated the C: drive, OS files, and other data from one hard drive to another, and another, and so on. Now, I have a 4TB SSD. I had an OS license until several years ago, when I upgraded my motherboard, CPU, and RAM. Windows required me to re-enter my license key, but I couldn't find it, which resulted in an unlicensed OS. I recently purchased a Windows 11 Pro key and upgraded the main components a few weeks ago. In trying to resolve the Windows Health Check error stating that my system does not support Secure Boot, I've attempted to use the CMD mbr2gpt command multiple times to no avail. The validate command runs successfully. I have activated the key in this same system, in case it somehow fixes the mbr2gpt error. My motherboard (ASUS TUF GAMING B650E-PLUS WIFI) does, in fact, support Secure Boot. When I try to update my BIOS settings by disabling the "Launch CSM" setting, it prevents Windows from loading to the Windows splashscreen. And so, I have to enable it again and make sure that the sub-setting "Boot Control Device" to at least "UEFI and Legacy OPROM". Windows won't boot if I set it to "UEFI only". The System Reserve has 100MB, the C: drive has nearly 1 TB, the Recovery Partition has 1.1 TB of space, and an unallocated section with 1.6 TB. The C: drive has around 112 GB of free space, more than enough for mbr2gpt to create the EFI System Partition.

What am I missing here? What do I need to do to convert the Disk 2 to GPT?

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Answer 1

Volume Z 1,390 Volunteer Moderator

Very good, Windows RE is enabled. If you run

reagentc /boottore

and reboot, you will be taken to Recovery Environment right away. Inside Recovery Environment access command prompt and run

mbr2gpt /convert /disk:2

without the allowFullOS switch.

Spencer Pack 30 Reputation points

2025-09-05T06:08:52.9066667+00:00

Success! Thank you so much. This was an easy resolution to the mbr2gpt error.
Volume Z 1,390 Reputation points Volunteer Moderator

2025-09-05T06:15:25.2466667+00:00

You're welcome. :)
Spencer Pack 30 Reputation points

2025-09-05T06:50:30.84+00:00
Synopsis and a bit more:

Make sure that the Recovery Environment is enabled. Run CMD command: 'reagentc /info'
The output will show if it is enabled. If not, add '/enable' to the command. You can type /help to get a list of all available commands if you would like to double-check.

Reboot your system.

When restarting, after the BIOS splash screen, you'll enter the WinRE (Windows Recovery Environment. Select Troubleshoot, then Advanced Options (I might have those reversed).

Now, click on Command Prompt. Enter 'mbr2gpt /convert /disk:2' without using the '/allowFullOS' switch.

Once that runs successfully, enter 'exit' to exit the command prompt. Then back out in the Recovery Environment to the option that allows you to "Continue" to your Windows system.

You're system will reboot. Make sure, at the BIOS splash screen, enter the BIOS screen by pressing the displayed key command(s). In my past experiences with different motherboards, you'll be prompted to press either the F2 or DEL (Delete) key, but that isn't a guarantee that your motherboard would require you to press a different key or keys.

In BIOS, set the Secure Boot setting to UEFI Only or similarly labeled options.

Disable the Launch CSM (Compatibility Support Module) setting.

Then, save and exit the BIOS.

The system will reboot. Then, once prompted, log into Windows.

Verify that the changes were successful.

Pull the Disk Management screen. Right-click on the disk "box", not the volume or drive, that has your C: drive. Click on properties, then, in the pop-up window, select the Volume tab. To the right of "Partition Style:" it should read "GUID Partition Table (GPT)".

Pull up the System Information screen. You can do this by entering 'msinfo32' in the command prompt (run as administrator). Check that the BIOS Mode says "UEFI". And, that the Secure Boot State says "On".
June Bug 0 Reputation points

2025-09-12T23:22:18.0766667+00:00

Just went through this and still had the message say, "Cannot find room for the EFI partition system."

I've been trying to get this computer I recently purchased to go through a factory reset and upgrade to windows 11. Any help on further steps?
Volume Z 1,390 Reputation points Volunteer Moderator

2025-09-13T09:53:02.9166667+00:00

Your System Reserved partition may be smaller than 100 MB. There may still be a solution, but please create a question of your own and additionally provide an image of disk management.

Answer 2

Volume Z 1,390 Volunteer Moderator

Try the following: Run MBR2GPT from outside the OS. No room for the EFI System Partition will have to be found as System Reserved will be converted to EFI. Do you have Recovery Environment enabled? Please post the output of

reagentc /info

Spencer Pack 30 Reputation points

2025-09-05T05:01:30.2366667+00:00

Here is the output from reagentc /info.

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Cannot find room for the EFI system partition. MBR2GPT: Conversion failed

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