Convert legacy bios to uefi on old fashioned install

I know to convert legacy bios to uefi using mbr2gpt IF the legacy bios has a separate system partition but it does not work for old fashioned legacy bios installs where there is not a separate partition.

I also know how to do it the older way with Macrium Reflect i.e. image backup C drive, clean install in uefi, overwrite C drive with image backup and use Macrium's fix Windows boot problems link.

Done that several times but always where the boot files are in a separate partition.

I want to try that with an old fashioned legacy bios install i.e. no system partition but I cannot work out how to actually create an old fashioned install in the first place.

Anybody know how to do this?

Edit - I am wondering if I create a drive with only one partition, and install to that, it would not create a system partition as normal.

Alternatively have four primary partitions (three small, one large) and then it is not possible to create a system partition?

Thoughts?

SIW2 said:

Mark the partition Active before installing.

Ah gotcha - thanks.

It turns out I was wrong - you can convert an old fashioned legacy bios installation (thanks @SIW2 for the tip that helped me).

I read the MS MBR documentation and it said you need a bit of space either side of the partition. I used minitool partition wizard free, and shrank the partition by 500 MB either side (far more than needed but just chose to be on safe side).


I then booted from a windows 10 1803 installation drive, pressed shift+f10 to get to command prompt and typed

Note: my OS was on drive 0

Code:

mbr2gpt /validate /disk:0 /logs:c:\

this validated fine, so I then typed

Code:

mbr2gpt /convert /disk:0 /logs:c:\

and it converted fine.

I actually did the above in a gen 1 Hyper-V virtual machine, and once converted, I created a new gen 2 vm (secure boot off) and attached the vhd from the gen 1 vm and it booted perfectly.


It did not actually change the unallocated space either side of the partitiojn but just shrunk c drive by 100 MB and added EFI.

SIW2 said:

As expected. Only an efi system partition and a windows partition are needed.

I thought it might have used the unallocated space as plenty but seemingly not.