Could be the jumper setting's on the hard drive, could be the IDE cable, could be the IDE controler on the motherboard.

1st thing to do is check that your BIOS picks up the hard drive. After that check the jumper setting's. Some hard drive's (IBM for example.) will have different jumper setting's for things like Master Single Device, Master with Slave Device. If you have a spare IDE cable try connecting that or try it with the IDE cable that is connected to your CD Drive. After that you can try switching the drive to the other IDE controller on the motherboard.