Convert Windows 10 from Legacy BIOS to UEFI without Data Loss  

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  1. danteafk
    Posts : 16
    Windows 10 x64
       New #180

    Hello,

    wondering if someone could help:

    I bought a new nvme ssd and did not remove the other disks while installing, thus had to enable CSM to get it installed.

    Now I wanted to disable CSM, and in order to achieve this (with the help of google) I used diskpart and the below commands to create a new EFI partition but used the wrong disk (the OS disk is MBR so I would have not been able to do that there anyway I think)

    Code:
    diskpart​list disk​
select disk 1​
(select 476GB disk)​
list partition​
select partition x​
(select xxxGB partition, x is 1 or 2)​
shrink desired=500​
create partition efi​
format fs=fat32 quick​
assign letter=H​
exit​
bcdboot c:\windows /s H: /f UEFI


    Now, I found this thread here which allows me to convert my OS Drive from MBR to GPT, however when I use it, it fails because obviously the EFI partition is somewhere else.

    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png

    What can I do? Does it matter that the EFI is on another disk? performance?

    Thanks
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  3. Brink
    Posts : 69,076
    64-bit Windows 11 Pro for Workstations
    Thread Starter
       New #181

    Hello @danteafk,

    Did you disconnect all hard disks other than the Windows disk when trying to covert it?
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  4. Paul Black
    Posts : 18,034
    Win 10 Pro 64-bit v1909 - Build 18363 Custom ISO Install
       New #182

    Hello @danteafk,

    danteafk said:
    I bought a new nvme ssd and did not remove the other disks while installing, thus had to enable CSM to get it installed.

    Now I wanted to disable CSM, and in order to achieve this (with the help of google) I used diskpart and the below commands to create a new EFI partition but used the wrong disk . . .

    Here is a bit of sound advice . . .

     Disconnect ALL Other Disks

    It is highly recommended that you disconnect [ temporarily unplug ] ALL other Disks [ HDD's/SSD's/USB's ] BEFORE installing Windows. The reasons for this are . . .

    • You CAN'T accidentally OVERWRITE a connected Disk.
    • The BOOTLOADER [ boot configuration files ] will get installed on the correct Disk [ because Windows has the tendency to install the bootloader rather randomly on ANY connected Disk ], and therefore STOP problems with booting once the installation is complete.

    Once the installation is complete . . .

    • Check that the OS boots correctly.
    • Check if the boot order in the BIOS/UEFI settings are correct [ if NOT, adjust them accordingly ].
    • Reconnect the other Disks.

    EXPLANATION:

    Windows uses a different partition structure for BIOS/UEFI. During the installation process, Windows asks which Disk / Partition you want to use for the Windows C:\ drive, it does NOT however, necessarily use the same Disk for the other Partitions, and can therefore incorrectly create the hidden recovery partition on ANY attached Disk.

    Additionally, for BIOS, if another Disk is left attached, and at a latter stage you remove that Disk, you will find that the OS will NOT boot. If you check the BIOS, you will see that the OS has assigned the AHCI/RAID setting instead of the LEGACY setting and set it up as RAID, therefore breaking the boot process.

    I hope this helps.
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  5. danteafk
    Posts : 16
    Windows 10 x64
       New #183

    Thank you both, in hindsight we'll all get smarter :)

    I've unplugged all drives but the OS, getting this now:

    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png

    How do I assign the recovery partition back to Drive 0 ? (if possible at all for uefi?)


    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png
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  7. Brink
    Posts : 69,076
    64-bit Windows 11 Pro for Workstations
    Thread Starter
       New #184

    danteafk said:
    Thank you both, in hindsight we'll all get smarter :)

    I've unplugged all drives but the OS, getting this now:

    How do I assign the recovery partition back to Drive 0 ? (if possible at all for uefi?)
    It shows the conversion was successful, and you will need to enable UEFI (disable CSM) in your BIOS settings for it to boot properly afterwards.

    If you have an issue booting while on UEFI, try performing a startup repair.
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  8. danteafk
    Posts : 16
    Windows 10 x64
       New #185

    I'm Back on UEFI thanks.

    But do I need to do anything with REAGENT because I have two recovery partitions now? One on disk 0 and one one disk 3?

    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png
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  9. Brink
    Posts : 69,076
    64-bit Windows 11 Pro for Workstations
    Thread Starter
       New #186

    danteafk said:
    I'm Back on UEFI thanks.

    But do I need to do anything with REAGENT because I have two recovery partitions now? One on disk 0 and one one disk 3?
    Great news.

    Personally, I wouldn't risk breaking it over a small partition.
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  10. Paul Black
    Posts : 18,034
    Win 10 Pro 64-bit v1909 - Build 18363 Custom ISO Install
       New #187

    Brink said:
    Personally, I wouldn't risk breaking it over a small partition.
    I agree.
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  11. danteafk
    Posts : 16
    Windows 10 x64
       New #188

    Cheers again,

    is there a way for me to see which recovery partition is active in use? I assume the one in Disk 3 because that's the one I've setup earlier today?

    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png
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  13. NavyLCDR
    Posts : 18,424
    Windows 11 Pro
       New #189

    danteafk said:
    Cheers again,

    is there a way for me to see which recovery partition is active in use? I assume the one in Disk 3 because that's the one I've setup earlier today?

    Convert Windows 10 from Legacy BIOS to UEFI without Data Loss-image.png
    In a command prompt with admin, or powershell with admin run:

    reagentc /info

    Find Reset Recovery Image Location in Windows 10
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