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mbr2gpt fails with "Cannot find room for the EFI system partition."

Anonymous
2021-09-27T15:11:02+00:00

Greetings. As Windows 11 comes close to it's release, I thought I'd start preparations in advance. My PC is fully compliant with Windows 11, however my disks all have MBR partition instead of GPT so I wanted to convert it as soon as possible. However, I hadn't had any luck with using mbr2gpt utility, it always fails with "Cannot find room for the EFI system partition." error. I searched for solution several times and yet to succeed.

In my PC I have two disks: with ID 0 is a RAID0 array (I think I have an Intel RAID controller in my motherboard, so it's not software RAID) of two HDDs, it's showing as a "boot drive", and with ID 1 is an SSD that is showing as "system drive". I have no clue why "boot" and "system" drive are separate and not sure if it's supposed to be that way.
I tried:

  • Defragmenting my SSD (it has 8% fragmentation, which is mostly in the middle of the drive, not in the beginning and not in the end)
  • Shrinking the volume on SSD to leave unallocated space for mbr2gpt to create EFI partition
  • Creating additional 500Mb empty volume for EFI partition
  • Tried running mbr2gpt from recovery menu (all other attempts were made with /allowFullOS argument) - still same error

I also had a Recovery partition on my system disk, which I removed (once again, that was one of the solutions I found). After that I started getting new error "Cannot fund OS partition(s) for disk X" (tried on both "boot" RAID array and "system" SSD).
In conclusion, I have two main questions:

  1. What might be the reason why I can't use mbr2gpt on any of my disks?
  2. If I manage to somehow switch from MBR to GPT, will my RAID array even work or will I have to backup all my data and then restore it with newly built RAID0? As far as I know, RAID works only for MBR and not GPT. I might be wrong so feel free to correct me, I would be grateful.

Any help or clues will be helpful, so thanks for anything.

Windows for home | Windows 10 | Files, folders, and storage

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  1. Igor Leyko 110.6K Reputation points Independent Advisor
    2021-09-27T16:13:53+00:00

    Hi Никита,

    My name is Igor, I have 12 Microsoft MVP awards. It's a pleasure for me to help others and I'll do all my best to help you. I'm sorry you have a problems.

    Boot disk is a disk the boot starts from, system disk is disk with installed Windows. You need to convert boot disk to GPT not system disk.

    Or you may reassign boot disk to SSD and repair boot.

    And see https://docs.microsoft.com/en-us/windows/deploy... for convertion instructions.

    Please note, you need unallocated space both at at the beginng and at the end of disk:

    ◦16KB + 2 sectors at the front of the disk

    ◦16KB + 1 sector at the end of the disk

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  1. Anonymous
    2021-09-28T20:50:49+00:00

    For future reference, when the system is converted, the C: partition is usually shrunk and the EFI System (Boot configuration files) is added in that space. If it could not find room, maybe it could not shrink the C: partition..

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  2. Anonymous
    2021-09-28T20:27:50+00:00

    Thank you very much for your response. I worked on solving my problem some more and checked some info about what you suggested. It appears to me that my boot configuration was messed up somehow after several system disk migrations and repairing it would take more time than to install fresh Windows and copy my files on the other drive. I will mark your solution as correct, because for other people with that problem it would probably solve it. Once again, thank you.

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