Convert Windows 10 from Legacy BIOS to UEFI without Data Loss  

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  1. Posts : 17,246
    Windows 11 Pro
       #120

    Since you have the Macrium Reflect image, then the MBR to GPT is reversible. From a command prompt in the Macrium Free rescue drive it would be:
    diskpart
    select disk 0 <- assuming the SSD is drive 0. You can run the list disk command to make sure.
    clean
    convert mbr <- probably unnecessary as the clean command automatically sets it to MBR
    exit
    exit

    Then restore the image to the SSD.
      My Computer


  2. Posts : 725
    Windows 10 64-bits
       #121

    NavyLCDR said:
    Since you have the Macrium Reflect image, then the MBR to GPT is reversible. From a command prompt in the Macrium Free rescue drive it would be:
    diskpart
    select disk 0 <- assuming the SSD is drive 0. You can run the list disk command to make sure.
    clean
    convert mbr <- probably unnecessary as the clean command automatically sets it to MBR
    exit
    exit

    Then restore the image to the SSD.
    Thanks NavyLCDR...

    The laptop has one SSD, but will verify it with the "list disk" in "diskpart". I will run the "convert mbr" command might as well, don't like to rely on "default"...
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  3. Posts : 13,536
    Windows10
       #122

    NavyLCDR said:
    @Cr00zng,

    Why do you want to convert the drive to GPT?

    Yes, you can reverse the process, but it requires an external drive to do so. You would have to create a backup image of the internal drive saved to the external drive with a program such as Macrium Reflect Free. Then you re-initialize the internal drive back to MBR, and restore the backup image to it.

    If you only want to enable UEFI booting, you can do that without converting the drive to GPT. You only have to recreate the System Reserved partition as a FAT32 system partition, and recreate the boot files in it.
    I do not follow. Surely Windows 64bit will only boot in UEFI mode if a GPT drive?
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  4. Posts : 17,246
    Windows 11 Pro
       #123

    cereberus said:
    I do not follow. Surely Windows 64bit will only boot in UEFI mode if a GPT drive?
    In post #117, @Cr00zng was asking about converting his drive from MBR to GPT and the possibility of converting it back to MBR. I then asked why they wanted to convert the drive to GPT. If the reason for converting the drive to GPT was only to be able to boot in UEFI mode, it is not necessary to convert the drive to GPT. UEFI will boot from either an MBR or GPT drive so long as the system partition is FAT32, not NTFS (and some computers will boot from an NTFS partition in UEFI mode, but that is not part of the UEFI specification).

    However @Cr00zng's reason for converting to GPT was more than just to boot in UEFI mode so it does sound like they need to convert their drive to GPT. And yes, a GPT drive does required UEFI to boot mostly because you cannot mark a partition as active on a GPT drive, which is required for legacy BIOS booting.
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  5. Posts : 13,536
    Windows10
       #124

    NavyLCDR said:
    In post #117, @Cr00zng was asking about converting his drive from MBR to GPT and the possibility of converting it back to MBR. I then asked why they wanted to convert the drive to GPT. If the reason for converting the drive to GPT was only to be able to boot in UEFI mode, it is not necessary to convert the drive to GPT. UEFI will boot from either an MBR or GPT drive so long as the system partition is FAT32, not NTFS (and some computers will boot from an NTFS partition in UEFI mode, but that is not part of the UEFI specification).

    However @Cr00zng's reason for converting to GPT was more than just to boot in UEFI mode so it does sound like they need to convert their drive to GPT. And yes, a GPT drive does required UEFI to boot mostly because you cannot mark a partition as active on a GPT drive, which is required for legacy BIOS booting.
    Have you ever actually tried booting an mbr full windows install as uefi? I know winpe drives can do it (or else mct could not be dual purpose)?

    I guess I will try it and see.
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  6. Posts : 17,246
    Windows 11 Pro
       #125

    cereberus said:
    Have you ever actually tried booting an mbr full windows install as uefi? I know winpe drives can do it (or else mct could not be dual purpose)?

    I guess I will try it and see.
    Yes, I have. I did a clean install to an MBR partitioned drive booting in CSM (legacy BIOS) mode. The drive would only boot in CSM mode. Then I deleted the NTFS System Reserved partition, created a FAT32 partition in it's place, marked it as active, and used the bootsect command to write BOOTMGR to the partition. Then I used the bcdboot command with the /f ALL switch to write the BCDs to the partition. The computer will now boot from that drive in either CSM (legacy BIOS) or UEFI mode. Each mode has it's own BCD in the partition, though. So if you modify the BCD in CSM mode, those modification will not affect booting in UEFI mode and you have to repeat the modification in UEFI mode.

    The drive in question is an SSD and it will boot in both modes when attached directly to a SATA port in my desktop computer or in a USB enclosure attached to my laptop.
      My Computer


  7. Posts : 1,324
    10 Pro retail 1909
       #126

    I have an Asus board z 390.
    I am in legacy and all is GPT.
    As an average user, I am having trouble moving to secure boot from legacy. The settings are not obvious unless you are a tech.
    My builder was not any help and I have tried the manual, forums and YouTube.
    Basically I wanted the steps to go from legacy to UEFI for security. I them wanted the steps to return to legacy in case I prefer it.
    Boards being different none wants to advise.
      My Computer


  8. Posts : 17,246
    Windows 11 Pro
       #127

    You can't be in legacy boot mode and be booting from a GPT drive. Legacy booting requires a partition that is set as active to boot from. GPT does not allow partitions to be set as active on the drive.
      My Computer


  9. Posts : 1,324
    10 Pro retail 1909
       #128

    new information


    NavyLCDR said:
    You can't be in legacy boot mode and be booting from a GPT drive. Legacy booting requires a partition that is set as active to boot from. GPT does not allow partitions to be set as active on the drive.
    >>I am in legacy mode.
    My goal was to change settings from legacy to pure uefi for security but more for the sake of learning..
    Then I wanted to know how to reverse this.
    Thirdly I was curious about booting from removable media.

    I opened disc management and under properties it says GPT Does this new information mean I cannot boot from a UEFI prepared pen drive?
    I did have success with macrium which is formatted for UEFI.

    PS - I was told that part of this process is to perform a clean install to get secure boot to be the default. Do you agree with that. I saw "default" in the settings that I thought would do if that was the case.
    Last edited by maranna; 20 Mar 2019 at 15:01.
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  10. Posts : 17,246
    Windows 11 Pro
       #129

    You may have legacy mode enabled - but you cannot actually boot in legacy mode from a GPT drive for the reasons I posted. If you think you booted in legacy mode from a GPT drive then run system information and BIOS mode will show UEFI.

    You cannot enable secure boot with legacy mode enabled. You must disable legacy mode first, then enable secure boot.

    If the disk properties for a drive show it as GPT then you MUST boot it in UEFI mode.

    You do not need to do a clean install to get secure boot enabled. It just has to be a genuine install of Windows 8 or Windows 10. But you must disable legacy booting first.

    Enabling secure boot is simply a UEFI firmware (mistakenly called BIOS) setting. It does not care anything about how the drive is formatted to enable it. But - if the drive does not have Windows 8 or 10 properly installed on it, the drive won't boot because secure boot is enabled.
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