Mac notebooks: About the Sudden Motion Sensor

Sudden Motion Sensor (SMS) 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.

All Intel-based Mac notebooks have Sudden Motion Sensor technology. These include:

• MacBook

• MacBook Pro

• MacBook Air

• PowerPC computers starting with PowerBook G4 (12-inch, 1.5GHz), PowerBook G4 (15-inch, 1.67/1.5GHz), PowerBook G4 (17-inch, 1.67GHz), and iBook G4 computers starting with iBook G4 (Mid 2005).

Note: Computers with Solid State Drives (SSD) or Flash Storage do not use SMS as the drives have no moving parts.

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.

What if I need to disable the Sudden Motion Sensor?

Apple has adjusted the Sudden Motion Sensor feature carefully to provide the best balance between protecting the hard drive and preventing unwanted activation of the Sudden Motion Sensor. Most Mac notebook owners never need to turn this feature off. Apple suggests that you not modify the settings unless absolutely necessary.

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.

First determine the status of the Sudden Motion Sensor

1. From the Finder's Go menu, choose Utilities.

2. In the Utilities folder, open Terminal.

3. When the command line appears, type sudo pmset -g and and press Return.

4. Type in the administrator password when prompted and hit Return. This command queries the computer for the current setting of the Sudden Motion Sensor. You can determine the setting by locating the Sudden Motion Sensor entry in Mac OS X v10.4 and later and looking to the right to determine its value. The default setting is "1" (turned on). If the sensor is off the setting will be "0".

To disable the Sudden Motion Sensor in Mac OS X v10.4 and later:

1. If the Sudden Motion Sensor is turned on, you can disable it by typing sudo pmset -a sms 0 into Terminal and pressing Return.

2. Type your administrator password when you are prompted.

3. Type sudo pmset -g command to be sure that the setting returns as "0".

Any changes that you make to the Sudden Motion Sensor setting remain in effect even after you restart the computer. If you choose to disable the Sudden Motion Sensor, Apple recommends that you enable it as soon as possible in order to take full advantage of the feature.

To re-enable the Sudden Motion Sensor in Mac OS X 10.4 and later:

1. If the Sudden Motion Sensor is turned off and the status returns a "0", you can re-enable the Sudden Motion Sensor by typing sudo pmset -a sms 1 into Terminal and pressing Return.

2. Type your administrator password when you are prompted.

3. Type pmset -g command to be sure that the setting returns as "1".