On Monday, June 27, 2011 04:52:52 PM John R Pierce wrote:
> On 06/27/11 1:40 PM, Lamar Owen wrote:
> > That would wipe more than the data; it would also wipe the embedded servo information and render the drive completely useless until someone with a servowriter ...
> I might be wrong about this, but I was led to believe that the servo
> data is written with special heads onto the platters before the disks
> are even assembled.
Servo can be written after assembly, through a 'window' in the side of the drive that is later covered with a 'warranty void if removed' sticker. But, as you say, it is done with special heads, on a servowriter. These are highly specialized and expensive machines, and require absolute positioner accuracy better than the track spacing of the disk (a laser interferometer is one way to do this good of positioning accuracy; we use a HeNe laser interferometer on our photographic plate microdensitometer system here, and it's good to one-tenth of a wavelength of HeNe light (63 or so nanometers)).
Writing servo on the individual platters, and then maintaining tolerance during platter stack assembly, would be very tricky indeed. Much easier to write servo on an assembled platter stack, to make sure the cylinders really are concentric.
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