have you put on the jumpers right? does it have any partitions? are any pins missing? was the cable incorrectly installed, bending or breaking a pin? If there isn't anything important on the drive, have you tried formatting it or any of its partitions? have you tried reinstalling drivers?
open the thing back up and see if u accidentally screwed up something or check if theres something shorting out any part of your mobo (this happens alot if you dont use lint free / tear free cloth)
AMD 64 3400+ s939 / ASUS A8V Deluxe (K8T800Pro) / Kingston Dual Channel DDR 400 (2 x 256) Corsair Value DDR 400 (2 x 512) / ATI Radeaon 9600XT / Seagate 120 GB/