Herringbone-grooved hydrodynamic bearings are now being applied to hard disk drive (HDD) spindle motors instead of ball bearings for demand for HDDs with larger storage capacities, faster access times, and reduced sizes. A clear understanding of the vibration characteristics of HDD spindle systems supported by hydrodynamic bearings is needed for the industry to achieve significantly better run-out characteristics. Such HDD spindles are well known to exhibit vibration mainly in rocking mode. From these backgrounds, the resonance vibration characteristics of HDD spindle systems with hydrodynamic bearings were investigated focusing on the rocking resonance mode of the spindles in this paper. To express the resonance vibration, a lumped mass model comprising five main components was used for the study. A comparison of the theoretical results with ones obtained experimentally demonstrated its validity, and each effect of springs and dampers connecting the lumped masses to frequency response of the spindle systems was comprehensively discussed.