Two hard disks shown on BIOS but not on the disk management ???

Hi, guys.

I bought a used desktop last night. It is a dell XPS 420 which has two HDDs (300gb each). Everything works fine but only one of the HDDs are accessible. When the computer is booted, the BIOS screen shows that there are two HDDs. The disk management shows only one of them.

Here is the pics:

http://i513.photobucket.com/albums/t336/hermtm2/photo_z...

http://i513.photobucket.com/albums/t336/hermtm2/photo1_...

Can you guys help me out?
By the way I am using Vista Ultimate 32bits.

Thanks,
Ryan.