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An analysis of three-dimensional, unsteady flow field is conducted in a 3.5-in, 7200 rpm hard disk drive (HDD) using FLUENT software. The analysis is carried out for the head stack assembly (HSA) positioned at the outer diameter (OD). A finite element model (FEM) of the HSA, built in ANSYS, is used for the forced vibration analysis. The flow field analysis yields the aerodynamic forces and moments which are applied to the HSA FEM. The effects of a damper plate and arm holes introduced to the HSA design are investigated. The analysis shows that a damper plate attenuates the root mean square (RMS) of aerodynamic loads acting on the HSA, resulting in a significant reduction of off-track displacement of the heads. It also shows that a weight-saving hole in the arm causes a complex vortex structure inside the hole. The vortex structure exhibits a forming/collapsing frequency of 7 to 9 kHz, which resonates with the structural modes of the HSA, causing additional off-track vibration of the heads.