3.5" floppy failure mode? Hub not attched to media.

On Sat, Feb 28, 2015 at 11:14 AM, Mike Stein <mhs.stein at gmail.com> wrote: Yes, it did fully closed-loop servo positioning, with embedded servo bursts in all inter-sector gaps. That's why they have not one, but TWO stepper motors for positioning, with a mechanical adder mechanism. The coarse positioning stepper doesn't have the resolution needed for track following. This can also be seen in their US patent 4,630,145: "As the track densities are increased, the misalignment error may approach or become larger than the track spacing. The noise-to-signal ratio then becomes very large and the desired signal cannot be read." "The second stepper motor 46 is stepped by track following control circuits operating in a closed loop." "It is yet another object of this invention to step the head towards the track centerline by a predetermined discrete fine increment equal to the maximum amount of track eccentricity per sector." They didn't do this stuff because it was cool and they wanted to make the drive more expensive. They did it because it was necessary to make it work reliably at all.