They use many diffrent kinds. Some may come with Western Digtal some may come with Segate or any other brand of hd. It just depends on what they have in stock at the time.
Mine came with a Maxtor. I've got a bit of a mix though. I've got the Maxtor that came with it and added a WD SATA drive inside, and a Seagate in a USB 2.0 enclosure. My Mac's diverse