A system for generating resonance mode information associated with a servo track writer system comprising a drive VCM, a drive arm assembly, an external VCM, an external arm assembly, and a mechanical connection between the external arm assembly and the drive arm assembly. The system of the present invention comprises a controller, a position sensor, and a processor. The controller is adapted to apply an injection signal to the drive VCM such that the injection signal causes movement of the external arm assembly through the drive arm assembly and the pushpin. The position sensor is configured to generate an output signal indicative of a position of the external arm assembly. The processor generates the resonance mode information based on the injection signal and the output signal.