How to get bootable VHD into BIOS boot menu?

I also said the usual setup is to have a separate esp (system ) partition.

So formatting the os partition ( which you keep calling C ) does not affect the separate system partition.

Have a look in diskmgmt and you can see the esp partition

I have colored them for you

red is where bootmgfw.efi and bcd live

green is where the operating system lives

OK, got it.

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When talking about removing C, I meant the whole m.2 drive with all 4 partitions. The vhdx will still have them all intact though.

if you do that the disk containing the vhdx will need an esp partition with the relevant boot critical files.

you havent supplied a diskmgmt screenshot. But if the other disk doesnt have an esp, it is quite easy to create one, or just clone the existing esp onto the disk containing the .vhdx file.

This may be the critical info I need. The disk holding the vhdx does not have a system partition. Only the vhdx on that disk has it. So the vhdx will not boot unless I create a system partition.

and you need to create the boot critical files on the esp and the bcd entry to boot the .vhdx.

The lazy way is create an esp partition on the disk with the .vhdx, then just copy the contents of the esp from your os disk into it

or clone the esp partition.

So how come the vhdx currently boots without a system partition on the disk it resides on? Is the answer because it's stored on the C drive system partition currently.

So how come the vhdx currently boots without a system partition on the disk it resides on?

Is this a wind up?

the bcd entry points at the disk and the partition. So the bootmanager on the os disk can boot something from a different disk.

Is the answer because it's stored on the C drive system partition currently.

I dont know what you are asking. What do you mean by it

according to info you have supplied, the bootmgfw.efi and the bcd store are on the os disk, if that was the question.

OK, that explains it and why I have to create a system partition on the disk the vhdx resides on if the disk that c drive is on fails. Thanks for your help. I'll let you know how it turns out.

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"It" refers to the vhdx boot information.

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OK, I plan to use these commands

diskpart
list disk
select disk 3 (or whatever it is)
create partition EFI
format quick fs=FAT32
assign letter=A
exit
bcdboot E:\Windows /s A: /f UEFI

Where E is my mounted VHD letter.
I think it should work.

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Since my H: is the one holding the VHD and it's already partitioned I've read I can use disk management to shrink it but it will shrink from the end of the volume. I believe the system partition I'm trying to create needs to be at the start of the volume. Hmm...how to do that?

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After more reading it looks like the efi system partition can reside anywhere says @NavyLCDR. I will try these commands (with my volume numbers) -

diskpart
select volume c
shrink desired=100
create part EFI
format fs=fat32 quick
assign letter=t
exit
bcdboot C:\Windows /s T: /f UEFI
diskpart
select vol t
remove letter=t
exit
exit