Seagate's solution in this prototype was to hybridize the storage with the addition of 8GB of flash memory. The vast majority of the time, the tablet is just running on flash, and the magnetic drive is powered off. If you want to play a movie, though, the drive will spin up, swap the movie onto the flash memory through a fast 6 gb/s SATA interface, and then spin down again. The upshot of this is that you have 500GB that you can access whenever you want, but you're not paying for it in battery life, because it's almost never running.
The hard drive in question is Seagate's impressively skinny "Ultra Mobile HDD," a five-millimeter-thick single system with 500GB of storage, robust power management, and drop protection