"... I'll have a 1.8 inch 200 PB hard disk for $35 twelve years from now."
That would be really cool...
Unfortunately, once the size of an individual magnetic domain:
-- -- Magnetic domain on Wikipedia:
-- -- -- -- http://en.wikipedia.org/wiki/Magnetic_domain
on the platter is shrunk to a certain point, the polarity/magnetic alignment of that domain can spontaneously change due to ambient kinetic (vibration, etc.) and thermal (heat/temperature change) effects.
Patterned media:
-- -- Patterned media on Wikipedia:
-- -- -- -- http://en.wikipedia.org/wiki/Patterned_media
will help keep the increasing storage density trend going for the near future, but eventually we're gonna have to move to a Spintronics-based technology:
-- -- Spintronics on Wikipedia:
-- -- -- -- http://en.wikipedia.org/wiki/Spintronics
like Racetrack Memory:
-- -- Racetrack memory on Wikipedia:
-- -- -- -- http://en.wikipedia.org/wiki/Racetrack_memory
to get to your 1.8-inch 200 petabyte drive...