<p>Planar velocity fields were experimentally studied in a 2.25-scaled hard disk drive (HDD) model containing an arm, two disks, a hub, and a shroud opening mounted in a non-axisymmetric enclosure, to clarify how arm insertion angle affects inter-disk flow structures. Two-dimensional mean velocity fields were measured using particle image velocimetry in five planar locations at a Reynolds number of 7.4 × 10⁴. The flow was observed to divide at a critical radius between solid-body rotation directed toward the hub edge and sheared flow developing toward the disk edge. In areas of solid-body rotation downstream of the arm, when the arm was inserted closer to the hub, it contracted while it spanned a broader area at deeper insertions. Locally accelerated flows were observed in the narrowed space between the hub and the arm, with their magnitude exceeding the local disk velocity for deeper insertion angles. Contrary to the axisymmetric case, upstream of the insertion, the flow decelerated toward the shroud opening. An implication of an axial centrifugal secondary flow existed in the mid-plane upstream and downstream of the arm insertion as the flow moved inward in the hub direction. Midstream of the insertion, flow disturbances resulted from momentum exchange between the flow from the side of the arm and the shroud opening. The flow exhibited different patterns in the inter-disk region caused by geometric variations and the arm insertion, generating wakes and interacting with the inflow from the shroud opening. Based on these findings, non-axisymmetric features significantly alter the internal flow structures in HDDs and can act as sources of unsteady, flow-induced vibration. The geometry of the inserted arm and the shroud opening play a critical role in controlling internal flow patterns, providing important guidance for optimizing HDD design to reduce vibration and improve operational stability.</p> Graphic abstract <p></p>