Have you made sure that the power and IDE cables are plugged improperly and that the jumpers on all of your IDE devices are set correctly?
If that doesn't work, try using another power connector to plug your hard drive to the power supply.
Okay, this is one for the books. Yesterday, I installed a new cd burner. Quite simple to do. In my desktop i have 3 hd's, burner and floppy. 2 of the hd's are for video storage and data. Main hd hs XP. I didn't like the performance of the new burner, so i removed it and replaced the older one. Never touching any other cables other than the 2 required for the burner..ide and power. This morning upon booting up, I noticed that post was not acknowledging one of my hd's. I did a copy of Disk Management and sure enough one is missing. Now, how can this be. I hardly used the hd except on rare occasions of viewing or storing some small files. I did setup and IDE configuration, nothing. Any ideas?