If UEFI is supported, then execute following from
elevated command prompt.
Regular command will give error on last step.
If you get any errors then stop immediately.
Last message should be "Boot files created successfully".
diskpart
list disk
select disk 1
(select 250GB OS drive)
list partition
select partition 2
(select 511MB partition, we're going to delete it to make space for new bootloader)
delete partition override
create partition primary
format fs=ntfs quick
active
assign letter=H
exit
bcdboot C:\windows /s H:
After this is done, boot into BIOS and change boot priority (HDD BBS settings), so 250GB drive is first in boot order.
Check Disk Management. "System, Active" partition now should be on 250GB drive.
then follow this:
https://it-infrastructure.solutions/how-to-switch-from-legacy-to-uefi-boot-mode-mbr2gpt-convertion/