There are many control methods to guarantee the robustness of systems. Among them, disturbance observer (DOB) has been used widely because it is easy to apply and the cost to pay is low due to its simplicity. Generally, the output signal of the system is required to construct a DOB, but in some systems, such as magnetic disk drive systems and optical disk drive systems, we can not measure the position signal but only the position error signal is available. So, in order to implement a DOB to such systems, we must use a error signal instead of output signal. We call it as a error based disturbance observer (EDOB) system. We show in this paper the difference between conventional DOB system and EDOB system, and the effectiveness of the proposed EDOB through simulations and experiments. Also, this paper proposes criteria to enhance the robustness of the EDOB system, and reveals disturbance attenuation property of the EDOB system. Finally, we propose a new method to improve the tracking performance and verify this through experiments for Samsung 12X DVD system.