Position sensors other than the read/write head are not embedded into current HDDs as they either do not possess nanometer sensing resolution or are too costly. Moreover, the "optimal" location for placing the sensors is still unknown. This paper studies the nanoposition sensing issues in HDD dual-stage servo systems. Using SSA (self-sensing actuation), the PZT element in the PZT suspension-based micro-actuator is used as a secondary actuator and a displacement sensor simultaneously. With the high SNR estimated PZT micro-actuator's displacement, a robust AMD (active mode damping) controller is designed to damp the PZT micro-actuator suspension's torsion modes and sway mode, as well as decoupling the HDD dual-stage servo into two distinct loops for individual sensitivity optimization.