It varies. Some people will have Seagates, others will have Hitachis, and others will have Maxtors. I have yet to see a Mac with a Western Digital drive as standard equipment, but I wouldn't count that out either. However, they might use a drive with 2 MB of cache. Almost all SATA drives come with 8 MB, but that doesn't mean Apple will include 8 MB. After all, they already purposely reduce the read speeds of their optical drives to 32x for CD-ROMs (even if the actual drive can handle 40x, 48x or 52x). Since the hard drive is a user-serviceable part, you might be better off saving your money and buying a new hard drive yourself. Apple's cost for upgrading is pretty steep, well above what it would actually cost to sell your old hard drive and buy a new one.