Are you sure it is not hidden?
If you are using XP go into control panel, admin tools and get the drive manager up.
Then find out the drive location, normally: C:
Then go to start > run and type: C:\ and open it up.
If it is inaccessable there is a problem.
Also, are you sure it is connected to a SATA board or have a controller card for it?
Are you booting from this drive? and is it shown in the BIOS?