Hi Ian - it sounds like you have two drives on your computer. The jumpers on the drives must be set so one of them is the MASTER and the other one is the SLAVE (Or Cable Select). If this isn't done you will get all kinds of errors - if they work at all. To change the jumpers you can look at the label on the drive and they usually show a diagram of where the jumpers should be placed. Unfortunately they may be different with different manufacturers. The jumpers are located next to the plug where the big ribbon cable attaches. The jumper is just a small plastic piece that is placed over two brass pins. If there's no diagram, look under the pin location for the letter (M)(S) or (CS).
This sounds like the problem you have. I'm not sure if you're familiar with working on the drives, it's quite simple and if you run into difficulty feel free to email me (shopbruce@gmail.com) and I'll try to explain it more carefully for you.
Hope this helps!
Bruce
June 2008