I am planning on doing this same system with my A1200. I got a little PCB that converts the IDE header from 44 pin to 40 pin, so I can have a 3.5 hd in the A1200. I will run the slave device as a CD-rom drive externally ( have to keep it close by becuase the cable is only like 2 ft long) and will power the CD drive via a small PC Power supply unit I purchased used for about $1.50 All you have to do to get the PC power supply to fire up when you plug it in is splice the green and grey wires before the motherboard harness and connect them on opposite ends of a toggle switch. (Follow me)
Hope it helps.
