Can be a lot of things :

Bad HD (can you try target mode ? i suppose you cannot as the keyboard is not responding) i don't know how hard is removing the HD from those !!

Bad HD connecting cable

The thing is you advise the keyboard is not responding but you managed to get into open firmware !!

Holding the ALT key pressed down should give you the available booting devices... can you try ?

Bad DVD drive

As you have several to play with, try swapping parts.