In this paper, a 3.5-in computer hard disk drive consisting of a rotating-shaft fluid dynamic bearing spindle motor and 4-disk spinning at 15 000 rpm under external linear vibration is analyzed by commercial finite element analysis software. Predicted frequency response functions of the head-disk off-track motion per unit input acceleration are compared to measured hard disk drive position-error signals on a shaker table. Both theoretical and experimental results show a huge disk pack radial motion near 1600 Hz under linear in-plane vibration. The mode shape analysis indicates that the large radial motion without noticeable disk bending is mainly due to the bending of the motor bracket and shaft. The characteristics of the high frequency radial mode are discussed with the variation of fluid dynamic bearing parameters with the verification of experimental results.