I'm not sure they still make these things the way they used to, but they
used to write a (so called) servo track on one of the disks with a known
pattern.  The electronics could tell when the head was moving from one track
to another, so the head assembly would position itself on the right area no
matter what the orientation.  There was some talk of buried servo (putting
the info in the data tracks) but not sure if they implemented it.  Haven't
worked in the area for a while.  HTH
George&Mary Skokan ([log in to unmask])

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