Basically, it means you've bought an IDE external enclosure. So make sure you buy a 3.5" IDE hard disk and NOT Sata hard disk. (SATA is newer and faster interface). You should have bought a SATA one becoz you're going to buy a new HDD and dun have one lying around.

But since the deed already been done, just buy a 3.5" IDE HDD, personnally I'll recommend Seagate...have 5 or 6 in various forms ...XBOX, HTPC, desktop etc...never failed even for years.

Just stay away from Maxtor.