A control method for improving tracking performance with respect to a reference trajectory has been developed. The proposed control system has an inner/outer control structure. The inner control system is the existing control system based on a conventional PID controller, which is designed in advance and is independent of the outer control system. In order to improve tracking performance, the outer control system is designed based on optimal preview control theory and is added to the outside of the inner control system. A polynomial trajectory is used as the reference trajectory and is fed to the outer control system. The reference trajectory is modified online to minimize tracking error. The modified reference trajectory is derived as a control input signal from the outer control system and is fed to the inner control system. The effectiveness of the proposed control method is confirmed using numerical simulations in a benchmark problem for hard disk drives (HDDs).