Hitachi have a created cheesy animation entitled 'Get Perpendicular' to explain their new perpendicular recording technology.

One of the key challenges facing the hard drive industry is overcoming the constraints imposed by the superparamagnetic effect, which occurs when the microscopic magnetic grains on the disk become so tiny that they interfere with one another, thus losing their ability to hold their magnetic orientations. The result is “flipped bits” – bits whose magnetic north and south poles suddenly and spontaneously reverse – that corrupt data, rendering it and the storage device unreliable and thus unusable.

Longitudinal recording, as its name indicates, aligns the data bits horizontally, parallel to the surface of the disk. In contrast, perpendicular recording aligns the bits vertically, perpendicular to the disk, which allows additional room on a disk to pack more data, thus, enabling higher recording densities. via. ramp