There are many control methods to guarantee the robustness of a system. Among them, the disturbance observer (DOB) has been widely used because it is easy to apply and the cost is low due to its simplicity. Generally, an output signal of the system is required to construct a DOB, but for some systems such as magnetic/optical disk drive systems, we cannot measure the position output signal, but only the position error signal (PES). In order to apply a DOB to such systems, we must use an error signal instead of an output signal. We call it the error-based disturbance observer (EDOB) system. We analyze the differences between a conventional DOB system and EDOB system, and show the effectiveness of the proposed EDOB through simulations and experiments. Also, this paper proposes criteria to enhance the robustness of an EDOB system, and reveals the disturbance rejection property of the EDOB system. Finally, we propose a new method of a double Q system to improve the track-following performance. This is also verified through experiments for a DVD 12/spl times/ optical disk drive system.