Abstract: Apparatus and method for writing servo information in a head disc assembly (HDA) comprising a head, a disc and a spindle motor for rotating the disc. The servo information is written by the head to servo fields on the disc. A positioning system positions the head relative to the disc and a control circuit controls the positioning of the positioning system as well as the rotational speed of the spindle motor. An electro-mechanical transducer outputs a signal indicative of vibratory response of the HDA as the disc is rotated and a computer is provided which instructs the control circuit to rotate the spindle motor at each of a plurality of rotational speeds while characterizing the vibratory response of the HDA using the signal from the transducer for each of the rotational speeds. An optimal rotational speed is thereafter selected and used to write the servo information, the optimal rotational speed minimizing variation in the placement of the servo information.