Question Not decting a bootable drive

[Numlock.]

I have made a computer and i have reused my old 250gb hdd and i installed a new ssd, i also have an external hdd. When i was in the bios trying to configure the boot device it did not detect a 'bootable device', i know i had windows 10 on one of the hdd's but i cant rememmber witch one. what should i do?

 

[Obakasama]

It's recommended to fresh install Windows, rather than try to use an old drive with Windows from a different computer, for a new computer to avoid potential complications.

If you absolutely must get this old drive working, and you don't know which it is, then you'll have to start disconnecting the other drives and see if the system picks up on the old boot drive.