The interaction between flow instability and elastic vibration of shrouded corotating disks is investigated. Simultaneous measurements of velocity fluctuation and disk vibration are performed in a 3.5-in hard disk drive, which was modified to ensure geometrical axisymmetry. The dependence on Reynolds number of velocity fluctuation and disk vibration is examined. Spectral analysis and cross-correlation measurements employing hot-wire anemometry reveal the wave number, the speed of rotation, the coupling of instabilities on both sides of a disk as well as the axial extent of the unstable flow structure. The role of flow instability on disk vibration is emphasized.