@mailpup
You deserve credit, not me, because I said it different to how I meant to say it. lol.
What I meant was, regardless of whether the OS is installed on a disk or not, it should still enter into BIOS. I'd expect problems identifying a disk that isn't connected properly (I'm not saying it was deliberate, it sometimes happens), but it should still enter BIOS.
I don't see how a disk would effect that from happening. I could be wrong however, but I do feel much like you, that something else isn't right.
EDIT:
@DarkWolfie
Have you had it apart other than the drive bay?
Maybe you disconnected something and haven't reconnected it?
The proceedure of removing, then refitting a disk shouldn't cause BIOS to stop - So I'd be tracing exactly what you did to see if something has come undone during your disk change.
Try resetting BIOS again.