Abstract: A head and disk assembly for a fixed disk drive including control electronics and a host computer associated therewith comprises a frame and a cover for providing an interior space enclosed from the ambient; at least one data storage disk mounted within the interior space; the data storage disk having a data storage surface defining a multiplicity of concentric data storage tracks for storing user data and further defining factory prerecorded track centerline correction information which may be periodically sampled, held and processed by the control electronics; a disk rotation motor for rotating the disk relative to the frame at a predetermined angular velocity; a mass balanced, direct drive rotary actuator for positioning a data read/write head relative to the tracks in response to positioning signals supplied by the control electronics; an optical polyphase head position transducer coupled closely to the rotary actuator and the frame for generating plural phase cyclical signals e.g.