Converting OLD Legacy bios pc to UEFI using mbr2gpt


  1. Posts : 15,442
    Windows10
       #1

    Converting OLD Legacy bios pc to UEFI using mbr2gpt


    Some very old legacy bios installations have the boot files and OS on same drive, although later legacy bios installations separated the boot files and OS files into two separate partitions. The more modern UEFI standard keeps the (EFI) boot files separate as well from OS partition.

    There are a number of reasons for updating to EFI, and MS supply a tool to do that (mbr2gpt). I have done this a number of times for the more conventional legacy bios partitions - in essence the tool overwrites the system reserved partition with EFI partition.

    What I was unsure about is if you could upgrade a really old (single partition) legacy bios partition to UEFI.

    So I thought I would give it a try in a Hyper-V virtual machine. So I created a gen 1 virtual machine (legacy bios) and installed Windows 10 but due to "brain not in gear", I installed the later two partition installation, so had to start over.

    So I reasoned if the vhd had three primary partitions on it already, it would have to create a single W10 partition with boot files on same partition as the OS as a 4th primary partition.

    Here I came across a quirk of disk management i.e. it always made the 4th partition logical rather than primary partition. Sorted this with minitool partition free.

    I then installed Windows 10 in Hyper-V on this 4th partition and sure enough - boot files and OS files in same partition.

    It was at this point I realised I had over complicated things. All you have do is set up vhd as mbr and only create a single partition(NTFS). So started over, and thatt worked fine.

    I then connected vhd to host pc and ran (my vhd was drive 2).

    c:\windows\system32\mbr2gpt /convert /disk:2 /allowfullos

    It was successfully converted to UEFI format. The single partition was slightly reduced and a new EFI partition added.

    Connected VHD to a gen 2 vm, and it booted fine.

    So this confirms mbr2gpt will convert the really old single partition legacy bios installations to UEFI.


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  2. Posts : 18,044
    Win 10 Pro 64-bit v1909 - Build 18363 Custom ISO Install
       #2

    Hello @cereberus,
    Interesting Post, and a very nice diagnostic and resolution exercise. Thanks for sharing.
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  3. Posts : 6,247
    Windows 11 Pro - Windows 7 HP - Lubuntu
       #3

    Some years ago I tried to convert legacy - MBR Win 7 to UEFI - GPT and for my surprise it worked (as Win 7 doesn't have mbr2gpt.exe I booted from a Win 10 installation drive, launched a CMD window and then converted the drive). So I suppose it also work for Win 8.x.
    On sevenforums.com I have successfully oriented people to convert their drives from legacy - MBR Win 7 to UEFI - GPT using Win 10 installation drives.

    Most legacy - MBR has a System (active) partition. Some OEM place their diagnostic and maintenance applications on this partition.
    What I have seen is that legacy - MBR System (active) partition isn't converted into EFI Fat32 partition. A new EFI Fat32 partition is created and the System partition remains (useless as OEM diagnostic and maintenance applications doesn't work anymore).
    Last edited by Megahertz; 07 Feb 2022 at 10:09.
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  4. Posts : 18,424
    Windows 11 Pro
       #4

    And it is also interesting to note that the MSR (Microsoft System Reserved) partition that Microsoft claims is needed on GPT drives is NOT created when using mbr2gpt. So much for it being a required partition...
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  5. Posts : 6,247
    Windows 11 Pro - Windows 7 HP - Lubuntu
       #5

    NavyLCDR said:
    And it is also interesting to note that the MSR (Microsoft System Reserved) partition that Microsoft claims is needed on GPT drives is NOT created when using mbr2gpt. So much for it being a required partition...
    Never seen a good explanation for the existence of MSR partition. On Win 7 it was only used if you had BitLocker (Win 7 Ultimate).
    As the UEFI protocol doesn't mention it, I think it is also used for BitLocker.
    I suspect it has the decryption key to be used by security agency s to easily decrypt your drive.
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  6. Posts : 18,424
    Windows 11 Pro
       #6

    Megahertz said:
    I suspect it has the decryption key to be used by security agency s to easily decrypt your drive.
    I like your conspiracy theory!
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  7. SPB
    Posts : 78
    Windows 10 Home 22H2 x64
       #7

    cereberus said:
    It was successfully converted to UEFI format.
    The single partition was slightly reduced and a new EFI partition added.

    So this confirms mbr2gpt will convert the really old single partition legacy bios installations to UEFI.
    Very useful information!
    So where did it create the new EFI partition, in front of the main OS partition or behind?
      My Computer


  8. Posts : 15,442
    Windows10
    Thread Starter
       #8

    SPB said:
    Very useful information!
    So where did it create the new EFI partition, in front of the main OS partition or behind?
    After if I remember but order is not important. You can always rearrange otder by making an image backup and testorung in different order.
      My Computer


 

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