Embodiments of the invention prevent offtrack write that will be caused due to a head vibration in a storage device. In one embodiment, a hard disk drive (HDD) judges approval/disapproval of write of user data to a magnetic disk by using a read signal for user data in addition to servo data in write processing. Thus, a head vibration which cannot be precisely detected based on servo data is detected to prevent off-track write. More specifically, the HDD acquires a read back signal amplitude of a read element with respect to a user data sector within adjacent servo data during a write access phase, and detects a vibration from a maximum value MAX and a minimum value MIN of the read back signal amplitude to judge data write approval/disapproval.