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This paper proposes a new nonlinear time-varying unified control scheme (UCS) for track-seeking and track-following in hard disk drives (HDDs) so as to avoid problem of mode switching. The proposed control scheme utilizes the concept of the conventional composite nonlinear feedback (CNF) scheme to achieve fast and smooth seeking. In order to be advantageous over the conventional CNF control, the UCS consists of both linear and nonlinear time-dependent components, which are independent of absolute seeking error. Simulation and implementation results show that, during track-seeking mode, the proposed scheme has a significantly better performance robustness against variations in seek length as compared to the conventional CNF control. For track-following, disturbance and noise models are involved in the problem formulation and sub-optimal H ?? method is used to design the controller so as to achieve high head-positioning accuracy. It turns out that the unified controller removes 27.8% of the nonrepeatable disturbances and noise.