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With increases in recording density and higher rotation speeds, the aerodynamic aspect of hard disk drives (HDDs) is now quite significant. We studied the internal airflow of HDDs using a direct numerical simulation. We simulated two cases: the first had an arm with no weight-saving hole (no-hole case), while the second had an arm with a weight-saving hole (hole case). We observed periodic vortices (Karman type) for which the frequency was about 4 kHz in the downstream of the arm in both cases. The scale of the vortex in the hole case was larger than that in the no-hole case. In the hole case, the spectrum of the arm torque had sharp peaks that would directly affect the head positioning accuracy. Moreover, we found that there was no remarkable peak in the no-hole case.