این هم مرتبطه: http://support.apple.com/kb/HT1935
The Sudden Motion Sensor is designed to detect unusually strong vibrations, sudden changes in position, and accelerated movement. If the sensor detects any of these, it instantly parks the hard drive heads to help reduce the risk of damage to the hard drive in case of impact. The parking of the heads may be accompanied by a small clicking or "tink" sound which is normal and expected. If you are actively playing video, audio, or performing other similar tasks, you may notice a pause in the task when the SMS engages. The task should resume as soon as the SMS is in a state where it can disengage.
When the SMS senses that the computer's position is once again stable, it unlocks the hard drive heads and returns to normal operation. In general, SMS protection does not change hard disk performance with regular operation. For best results, avoid picking up or moving the computer off your work surface while it is awake and active. If you must move it, sleep it by closing the lid or using the Apple Menu.