the linear motor should have had a speed limiter and a cut out that operated as soon as the linear motor had rotated enough times (if it was geared) or beyond a certain angle (if it was direct drive) that the head would be off the disk or squashed up against the spindle.
A linear positioner in those disk drives is not something that rotates, and it's not geared either. It runs on rails or sliders, driven by a voice coil enclosed in a magnet. Not unlike a loudspeaker. The limits are stops on the rails of some sort, usually its mounting points.
And if the feedback is knackered, there's no way the controller can tell the positioner is moving too fast or past its range.