Moving sata win10 disk from homebrew to dell 9020 - 'boot failed'

My wife's homebrew took a lightning strike. Motherboard cooked, system wont power up.
Bought a dell 9020 Optiplex, it boots its own drive win10 fine
Tested 2 drives, they are fine, plugged into my new dell, seen all works.

Trying to boot the homebrew win10, plugged into first sata (sata0), gets message 'System drive boot failed, hit any key to reboot'

If system boots the drive that came with unit, it sees the win10 homebrew disk and all the files just fine.

The standard disk boots in ata mode, changing it to ahci gives error halfway into boot with 'Inaccessible boot device', kind of expected

The homebrew disk gives error message about 'boot failure' that displays whether ata or ahci mode enabled

Anything simple I am missing? I have not yet tried uefi? mode. And I cannot remember what sata controller mode it was built in the homebrew

Legacy BIOS v. UEFI maybe? Old computer was legacy BIOS, new computer is UEFI?

Actually the homebrew is newer and the dell is 2+ yrs old. Dell is ata legacy mode

Dont remember how i setup homebrew

I tried changing dell to uefi but it says 'no file system found' and shows no boot devices

I would install Marcrium Reflect Free and make a Macrium Rescue Drive (USB flash drive). Do a clean install of Windows 10 to a hard drive in the new computer (or use one of the already booting Windows 10 hard drives, if you have nothing on it that you need to keep). Once you get a hard drive that boots in the new computer, then use the Macrium Rescue Drive to clone just the Windows 10 partition only from the old computer's HDD over the top of the Windows 10 partition on the new hard drive, replacing the new partition with the old partition. Then run the utility under the restore menu of Macrium Refelct to Fix Windows boot problems.

If the computer boots then into Windows 10, the problem was with the boot files on the old hard drive. If the computer stops booting from the hard drive, then the problem is with the Windows 10 installation and you likely won't be able to use it in the new computer.