This paper presents a unified approach to track seeking and following control for a dual-stage actuator (DSA) hard disk drive (HDD) system. Based on a doubly coprime factorisation (DCF) method, the DSA controller and the closedloop dynamics are expressed explicitly in terms of two design parameters. This greatly simplifies the optimisation of design parameters in meeting desired specifications. We then address how to use the design parameters to deal with specific problems in the DSA, that is, control allocation for disturbance rejection and trajectory planning for track seeking. Simulated results are also presented to verify the effectiveness of the proposed DSA controller. Compared with previous works, the proposed approach can fulfil track seeking and following tasks by a single controller without switching. Moreover, the unified controller can achieve desirable performances for both tasks that are equivalent to that by two separate conventional controllers.