You can buy any external drive (2.5 or 3.5 with case) as long as it has an USB connection to it.
Shop around the internet then you'll find plenty of them. Hear is an example http://www.xpcgear.com/externaldrives.html
http://www.tigerdirect.com/applications/category/category_tlc.asp?CatId=24
Happy shopping!
I have two 2.0 USB ports. What other specs should I take into account when shopping for an external? I have an older Presario.
thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic