The amount of precision required is akin to flying an F15 figher jet across the country, one foot off the ground, without crashing.
All the more amazing when you see how they test HDD for robustness. Imagine standing the thing on in edge & letting it fall flat while it was reading. When I used to buy them (5 or 6 years ago) this kind of 'drop testing' was the standard.
I'm sure the testing has only become more rigorous since.