The Sudden Motion Sensor is designed to protect your Macs hard drive in the event of a computer being dropped or an unusually strong vibration. Essentially what it does is park the hard drive head when movement is detected, which prevents it from potentially scooting across the disk surface and scratching or otherwise damaging the drive or drive head.
Generally speaking, you will want to always have the SMS sensor enabled, but Apple mentions that certain environments are susceptible to unnecessary drive head parking due to the SMS. Basically the SMS detects a strong vibration and then the hard drive parks which can cause video and music playback issues, amongst other annoyances. This is particularly true with concert halls with strong acoustics, recording studios, dance and night clubs, and even walking workstations (the ones with a treadmill under a standing desk). Also, some owners of SSD drives may want to disable the feature.
Disable Sudden Motion Sensor on a Mac Laptop
This works to disable the Sudden Motion Sensor on the MacBook Pro, MacBook Air, MacBook, PowerBook, and iBook running Mac OS X 10.6 and lower:
Type the following at the command line: sudo pmset -a sms 0
Hit return and enter your password
The SMS sensor is now disabled, it’s just as easy to reenable again by changing the zero to a one when you need the protection back:
Enable the Sudden Motion Sensor on a Mac Laptop
This works on the same hardware as disabling the feature, and it’s basically the same commands:
Type the following at the command line: sudo pmset -a sms 1
Hit return and enter your admin password
You’ll notice the commands are identical except that the -a sms flag now has 1 attached to it rather than 0 (standard computing protocol of 1 for on, 0 for off).
Checking the status of the Sudden Motion Sensor
If you’re not sure whether the motion sensor is enabled or not, you can check quickly with the command line:
At the command line, type: sudo pmset -g
Hit return, enter your password, and look for “sms” in the list. Seeing a 1 next to sms indicates that the motion sensor is enabled, seeing a 0 next to sms indicates the motion sensor is disabled
Most users will never need to adjust the sudden motion sensor, but if you do find yourself in a situation where there are continuous vibrations or movements and your Mac is behaving strangely, this may be the culprit.