Windows 10: Convert Windows 10 from Legacy BIOS to UEFI without Data Loss  

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  1.    18 Aug 2018 #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 ComputerSystem Spec

  2.    19 Aug 2018 #121

    NavyLCDR said: View Post
    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"...
      My ComputerSystem Spec

  3.    19 Aug 2018 #122

    NavyLCDR said: View Post
    @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?
      My ComputerSystem Spec

  4.    19 Aug 2018 #123

    cereberus said: View Post
    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.
      My ComputerSystem Spec

  5.    19 Aug 2018 #124

    NavyLCDR said: View Post
    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.
      My ComputerSystem Spec

  6.    19 Aug 2018 #125

    cereberus said: View Post
    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 ComputerSystem Spec


 
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