2.5" hdd recognized as dvd?

I found an old 2.5" drive and bought a new enclosure for it since the old one seemed to have crapped out. After putting it together and plugging it into my vista system, the drivers installed automatically. The only problem is that my computer thinks it's a dvd drive. I also tried it in my XP system and I got the same problem. It's a Toshiba MK4019GAX 40GB. The only explanation I can think of is that perhaps it was formatted for a different OS, making it unreadable. Can anyone explain this to me?