*edit* I dont think this is your problem, but i will leave it here for anyoen else [your reply to pelvis's post was no there when i started typing] *edit*
Most high capacity hard drive's can have a 'capped' so that they can be used with a motherboard/operating system that cant cope with the size of the storage device. The 'cap' is set by a jumper on the hard disc, the intructions on how to set it are normally printed on the top of that hard disc.
I would make sure that the 'cap' is not enabled, If it is then you will have to backup all of your documents as you will need to reformat the disc after you have removed the cap.