Hard Reading Error

I have a 20gb hard drive installed on a windows server 2003. I thought the hard drive had failed completely because i turned the server on one day and nothing. I bought a new hard drive and installed in but left the old one still in the ide chain. Apperently one the first 18gb are not showing up but I can still format and use 2gb of the drive. It will on show 2gb free and 18gb as used but I can’t access or reformat the 18gb of space. Any ideas of how I can completely regain the other 18gb of space?