The operation of propagation self servowrite can be improved by using
spirals written upon the disk. The spirals can be used to reduce the DC
drift of the propagation of the self servowrite. The writing of the
spirals can be improved by using the back EMF signal for the VCM to
control the velocity of the actuator during the writing of the spirals to
produce a more consistent spiral shape. Additionally, back EMF crossings
of the spindle can be used to space the spirals evenly about the disk.