A high performance, low cost rotating disk data storage subsystem includes a head and disk assembly. A plurality of sectors in data tracks include a servo portion containing a prerecorded pattern of a plurality of repeating, time staggered, non-phase-coherent bursts, each burst having a predetermined integral fraction radial offset relative to the other bursts and having a constant frequency preamble field followed by a unique value field related to its burst spatial location. A head is positionable relative to the tracks in response to information derived from a servo portion, a programmed microcontroller supervisor for receives operational data storage and retrieval commands from a host system with which the subsystem is operationally associated and controls the position of the head during track seeking and following operations in response to servo information read by the head from each servo portion and further in response to the commands, an interface under the control of the microcontroller supervisor receives the commands and data blocks from the host system and for sending commands, status words and data blocks to the host system, a data sequencer and memory controller under the control of the microcontroller manages flow of data blocks to and from the disk via the head and stores the information read by the head means from said unique value fields for subsequent processing by the microcontroller. A servo burst pattern providing phase quadrature and absolute position information, a servo writer, a servo qualification area and a timer, a media defect management method, an amplitude measurement circuit, and a time division control program for the microcontroller are aspects of this invention.