Abstract: A positioning mechanism for a magnetic fixed (or hard) disk storage unit incorporates a bearing block forming part of the magnet system for the rotational positioner, and carrying a positioning shaft. A pivot member rotationally seated on the shaft has pivot arms carrying magnetic heads and brackets for carrying the magnetic coil for positioning the heads. A pulse operated detent magnet is provided and incorporates a stop armature. The armature is biased into detent retaining position, but is restrained by retaining spring in mechanically self-holding condition. It is automatically released when the pivot member swings into the detent position, corresponding to the landing zone for the magnetic heads.