From an arti­cle dis­cussing Seagate’s plant in Ire­land (where 80% of the company’s read/write heads are pro­duced): an impres­sive anal­ogy of a HDD’s read/write head.

The dimen­sions of the head are impres­sive. With a width of less than a hun­dred nanome­ters and a thick­ness of about ten, it flies above the plat­ter at a speed of up to 15,000 RPM, at a height that’s the equiv­a­lent of 40 atoms. If you start mul­ti­ply­ing these infin­i­tes­i­mally small num­bers, you begin to get an idea of their significance.

Con­sider this lit­tle com­par­i­son: if the read/write head were a Boe­ing 747, and the hard-disk plat­ter were the sur­face of the Earth:

- The head would fly at Mach 800
– At less than one cen­time­ter from the ground
– And count every blade of grass
– Mak­ing fewer than 10 unre­cov­er­able count­ing errors in an area equiv­a­lent to all of Ireland.

via Kot­tke