To meet this demand, the areal density (i.e. the number of bits that can be stored per unit area) of magnetic hard disk data storage devices (HDDs) has been increasing rapidly year on year since 1991. [0003] Recently, companies such as Read-Rite and Fujitsu have demonstrated that HDDs can be constructed that provide storage densities of 50 Gbit/in2 and above, and it therefore seems likely that an