I just gutted a disk drive, and some statistics are as follows:
It’s a Seagate ST336607LC – a 36Gb disk – containing a single platter, with (obviously) two faces, top and bottom. There is a hole in the middle of the platter, where the motor drive / axle resides.
$pi = 3.14159265358979; # constant
$nfaces = 2;
$platterdiam = 83; # mm ; diameter == 2*radius
$axlediam = 30; # mm
$megabytes = 36 * 1000; # 36GB in decimal megabytes
$surfacearea = (($pi * $platterdiam) – ($pi * $axlediam)) * $nfaces;
$mb_per_mmsq = $megabytes / $surfacearea; # == 108.105244364306
So, each square millimeter of disk surface is 108 megabytes, and a 1mmsq section through the disk would provide two faces, or 216Mb.
Interesting – and disks are much bigger now, of course.
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