A disk drive is disclosed including a disk having a plurality of servo sectors that define a plurality of servo tracks, an actuator arm, a head attached to a distal end of the actuator arm, a mechanical position sensor operable to detect an estimated position of the actuator arm, and a voice coil motor for rotating the actuator arm about a pivot. The disk drive further comprises control circuitry operable to process the servo sectors to generate a control signal applied to the voice coil motor, detect an estimated velocity of the actuator arm, detect a runaway condition in response to the estimated position of the actuator arm and the estimated velocity of the actuator arm, and adjust the control signal applied to the voice coil motor if the runaway condition is detected.