You might need to update your bios on your motherboard. There is a limit to what size the bios can see. There are a couple utilities that you can use that probably come with your hard drive - ezbios is one of them that can get around it. What kind of motherboard are you using? What brand is the hard drive? Is it Serial ATA or Parallel ATA?
You'll never get ALL the space out of the hard drive due to some overhead, but I think you should be getting more than that..