Unsteady airflow in an actual hard disk drive (HDD) which has a rotary actuator is numerically simulated by using LES (Large Eddy Simulation) technique, and the aerodynamic force affecting on head gimbals assembly (HGA) is estimated from the simulation.  Then calculated force is fed to a structure analysis to predict the airflow-induced vibration of HGA.  Lastly, we calculate the head positioning error or the head displacements for the different HGA orientations to explain and classify the mechanism of airflow-induced positioning error (i.e. windage) in HDD.