A disk drive with means to prevent accidental landing of a head/slider on the disk surface is described. In one embodiment a shock sensor is provided to sense external shocks received by the mechanism. During loading (positioning) of the actuator bearing the head/slider over the disk, if a shock is sensed, loading is stopped and an immediate unloading is performed to return the actuator to the withdrawn position. In another embodiment the unload operation is performed when the rotational speed of the disk is changed. In another embodiment the unload operation is performed when the rotational speed of the disk drops below a threshold. A circuit and method for detecting when the rotational speed of the disk drops below a threshold is also provided. The unloading in the various embodiments may be performed using velocity control or without velocity control.