Drive not recognized by XP Pro

I have a friend that just added a disk drive and installed XP Pro on it. After Windows was loaded, Windows saw the old drive but did not let him access it in Explorer. He has about 25 Gig of information on it. How can he get to the information without deleting the stuff on the drive? I told him to go into disk management and tell windows to see the drive. Windows sees the drive as active, but he’s got nothing in Explorer. Ideas??? Thanks…….