Please help with Dual Boot on Windows

[Rizzler]

I have 2 hard drives running different versions of Windows 10. Both are basic MBR discs. I can use dual boot on Windows to boot into either. If I unplug disc 2 I can boot into disc 1. If I use only disc 2 it won't boot even if I set it as the default. I've changed the boot order etc in UEFI but same issue. I tried creating a boot partition by cloning the boot partition from disc 1. No luck.
Can anyone tell me what to do to make disc 2 bootable on it's own without disc1 attached? Any help would be much appreciated. Thx

 

[Rizzler]

Seems there are not many boffins on this site anymore. I managed to fix it myself. I kept drive 2 plugged in then booted from USB drive with Macrium Reflect PE and used the "fix boot issues" button. It rebuilt the Master Boot Record and boot files in a few seconds. Problem solved.