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Sensitivity function or error rejection function is crucial to determine the disturbance attenuation capability of a closed-loop control system. A nonlinear least-squares optimisation method is applied in sensitivity function shaping to design a feedback controller for a micro-actuator such that its sensitivity function fulfills desired specifications in order to attenuate disturbances at specific frequencies. The micro-actuator is used as a secondary actuator to enhance the positioning accuracy of a servo track writer for hard disk drives. The disturbance attenuation capability of the designed control system is also evaluated for disturbances with varying frequencies. Simulation and experimental results show the effectiveness of this method in designing a controller so that specific conditions imposed on the sensitivity function are satisfied.