hmmm....

http://www.pcguide.com/ref/hdd/geom/dataPRML-c.html

So, hard disks now use ADCs and DSPs to detect ones and zeros...

This looks like a job for ANAQUAD!

Is it too much to ask that two heads on opposite sides of the platter (or maybe two on the same?) stay perfectly-aligned over time and temperatures? 'Cause a rough estimate using anaQuad would easily double, plausibly even quadruple, the capacity despite using twice the physical space...

Thanks @Starhawk for sending me down that path, looking up ZIP drives' encoding-schemes!