As hard drive storage has started to dramatically increase, now with up to 500GB on a single drive, manufacturers have had to research alternative methods of storage to yield these outcomes. Traditionally more storage meant more platters, and not touching the platters' density. Take for example my favorite hard drive, the Western Digital Raptor. It first debuted as a single platter 36.7GB 10,000RPM disk. Then a few months later, a 73.4GB Raptor was released. How do you think they did that? They added an extra platter. Now just recently, a 150GB raptor was released. You get the idea; this one had four platters.
At that rate, there is no way you could have possibly created a 500GB hard drive at roughly 40GB per platter, without changing the form factor. However, thanks to a fairly recent technology named Perpendicular Magnetic Recording, such massive hard drives are actually feasible. PMR technology increases the density of the platters by packing more information in a tighter space. Think about the difference between CDs and DVDs. CDs store data in a track which has a pitch or width of roughly 1600 nanometers, where as DVD data uses a 740nm track. The same goes for PMR-enabled hard drives; they use a more precise head to read data that is packed into a smaller area. The increased density actually improves transfer speeds as well. The rotation speed of the hard drive remains the same, but since the density is increased you can achieve sizeable performance enchancements.
PMR has slowly been entering more mainstream hard drive markets. There is a possibility the hard drive you are using right now has PMR technology. This is one of the great technologies that gets overlooked in the huge industry switch to high definition media. I say it won't be more than five years before every new computer sold will sport a hard drive with PMR. The hot ticket item right now is the HTPC, demanding the most storage space possible, and you can imagine how much influence they have on the tech industry. PMR technology even has a place in extremely small 1 inch and 1.8 inch hard drives that can only have a one or few platters. The next iPod could sport PMR as well!