Possible ID conflict. Look up the jumper setting on your hard drive by doing a search for it at the manufacturers web site. If using two hard drives set one to master and one to slave. With just one hard drive you can use cable select but I prefer setting it to master. Fix the hard drive first and then connect the optical drive. The optical drives also have jumper settings. Check the ribbon cable into the logic board as that sometimes gets pulled out some when working on the hard drives.