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The Sudden Motion Sensor (SMS) is a type of active hard-drive protection developed by Apple, used to protect hard disk drives in their notebook computers. The system first appeared in 2005 on the refreshed PowerBook line and is present on all of Apple's non-SSD notebooks since then, including the MacBooks. The last MacBook with HDD was released in 2012.
With a triaxial accelerometer, the shock detector 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. A clicking noise can be heard when the sudden motion sensor activates.
Broadly speaking, there have been two types of Sudden Motion Sensor. The sensor used in the G4-based laptops resolved shifts of 1/52 g (e.g. the dynamic range was close to 6-bit), while the sensor used in the later Intel-based laptops have an 8-bit resolution (250 scale divisions). In at least one model of Intel-based laptop, the MacBook Pro 15", Apple uses the Kionix KXM52-1050 three-axis accelerometer chip, with dynamic range of +/- 2g and bandwidth up to 1.5 kHz.
Among the MacBook and MacBook Pro community there have been several owners who installed aftermarket hard drives already equipped with anti-shock features who reported experiencing kernel panic errors whenever their unit was physically moved. This is believed to be due to a conflict between SMS and the new drive's anti-shock function. The Western Digital Scorpio series of notebook hard drives have been the most frequently reported as being susceptible to this problem. In practically all cases, disabling SMS alleviated this problem without any negative performance impact.