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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.