Installing a combo drive?

I just recently got hold of a laptop combo drive (internal) and had fitted it inside my iBook G3 (dual USB) it fitted in perfectly, then when I go to boot my hard drive isn't recognised, I put the old CD-Rom Drive back in and it is working perfectly? What is going on?

have you ever thought that maybe the combo drive was no good? It could have been a loose connection.. I'd try putting it in again. When you say it isn't reconized, do you mean when you put a disk in nothing happens? If this is the case, check the system profiler and see if the optical drive even shows up.

Well basically I put the disc drive in and the folder with the question mark thing pops up when i turn it on. The disc drive is an apple disc drive but im not sure why my computer won't startup.