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I believe it is a Hitachi, probably the basic model, although for some reason mine has a Fujitsu drive that came with it.

EDIT: Ha, it appears that there really isn't a whole lot of consistency
 
Thanks for the replies; debating what HD to replace the stock 5400 with (already have a enclosure coming to slap the stock HD in). Right now I'm leaning towards the WD Scorpio Blue 5400/500GB or the WD Scorpio Black 7200/320GB.
 
The Scorpio Black is noticeably noisy 🙂 I upgraded my stock HDD with a Hitachi TravelStar 500GB 7200rpm and very happy with it.
 
I upgraded to a Hitachi Travelstar 320gb 7200rpm drive. I converted the stock drive to a Time Machine backup.
 
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