Sudden Motion Sensor technology is built-in protection for the hard disk that is designed to help prevent disk issues if the computer is dropped or undergoes severe vibration. The Sudden Motion Sensor is designed to detect unusually strong vibrations, sudden changes in position or accelerated movement. If the computer is dropped, the Sudden Motion Sensor instantly parks the hard drive heads to help reduce the risk of damage to the hard drive on impact. When the Sudden Motion Sensor senses that the Apple portable’s position is once again stable, it unlocks the hard drive heads, and you are up and running within seconds.

In general, Sudden Motion Sensor protection does not change hard disk performance with regular operation. Occasionally, the Sudden Motion Sensor may be activated by unusually strong vibrations at a time when your portable device is writing data intensively, such as when recording video or audio. In some environments, such as live concert halls, recording studios, or dance clubs, external vibrations may be major enough to cause the module to unexpectedly park the hard drive heads, resulting in interrupted sound or video playback. In these situations, you may find that you want to disable the feature temporarily.

The Sudden Motion Sensor (SMS) is Apple’s patented motion-based hardware and data-protection system used in their notebook computer systems. With a triaxial accelerometer, the system detects sudden acceleration, such as when the computer is dropped, and prepares the relatively fragile hard disk drive mechanism for impact. The system disengages the disk drive heads from the hard disk platters, preventing data loss and drive damage from a disk head crash. When the computer is stable, the drive operates normally again.

The motion sensor can also be read by programs running on the Apple portables and a number of programs exist that use the motion sensor to make the computer tilt-aware. This way, tilting can be used as a human interface device for instance for scrolling or for controlling games.