The first thing that I would be looking at is to change the BIOS Battery they do go flat and cause some really screwy problems when they are anything but working perfectly.
That will at least remove the possibility of a BIOS Problem not allowing the HDD to be read properly. If it still doesn't work after changing the battery and resetting BIOS you'll need to start to perform some hardware tests the first would be to download WD's Testing Utility available here
http://tinyurl.com/97zw7
If the drive comes up as faulty test it in another computer and see if it continues to come up as Bad. If it doesn't you'll have to look at either the M'Board or the Data Cable being the problem and occasionally the Power Connector or Power Supply being at fault as these can fail at any time or fail to produce the required power.
If the drive shows up as Good after the test you can then try some more invasive diagnostic tools available on the Ultimate Boot CD available from here
http://tinyurl.com/3jnpy
But before you start do the obvious things check every connection and make sure that it's properly inserted, check the jumper settings to make sure that the M'Board is properly configured and also check the BIOS Clear Jumper to make sure that it's not in the [b]Clear[/b] position.
Look at all the power connectors particularly the one that goes onto the HDD and make sure that the connectors are not loose. If you need to tighten up any connectors remember to unplug the system and allow the capacitors to discharge or you run the risk of destroying the Power Supply while you are fixing a minor problem.
Col
