Abstract: In a magnetic disc data storage device using exchangeable magnetic discs for recording data signals, a track containing reference signals is provided and a read-write head is moved to the reference track and as the disc rotates, the position of the head relative to the axis of rotation of the head is sampled. Any errors in the position of the reference signals due to eccentricity of the track are stored and are used during subsequent accesses of data tracks on the disc to modify the head position so as to ensure that the head follows a track having a predetermined spatial relationship with the reference track.

Abstract: In a magnetic-disc data storage device using exchangeable discs for the recording of data on circular tracks, a reference track is provided on the disc and during interruptions in the transfer of data, a read-write head is moved to the reference track and the error in its expected position, due to dimensional changes in the disc or the storage device or due to misalignment of the head, is measured. In subsequent data tracks access the position of the head is corrected by error measured so that the head is correctly aligned with the selected data track. Measurement of the error is carried out more frequently when the error is changing rapidly.