How to Manage the Sudden Motion Sensor (SMS) on a Mac

Enable or disable the SMS using Terminal

Apple Mac computers have a motion-based data protection system called Sudden Motion Sensor (SMS), which is used to protect a device's internal hard drive. The SMS uses motion-detecting hardware in the form of a triaxial accelerometer to detect movement in three axes or directions.

How Does SMS Work?

The SMS allows any Mac to detect sudden motion that would indicate the device has been dropped, knocked over, or in danger of a severe impact. Once this type of motion is detected, the SMS protects the Mac's hard drive by moving the drive's heads from their current active location over the spinning magnetic disk platters to a safe location retracted into the side of the drive mechanism. This is commonly referred to as "parking the heads."

With the drive's heads parked, the hard drive can endure a significant blow without suffering damage or loss of data. When the SMS detects that a Mac has returned to a stable condition, it reactivates the drive mechanism.

The downside is that the SMS may sometimes experience false trigger events. For example, if you're using a Mac in a noisy venue with enough low-frequency energy to vibrate the device, the SMS may detect these motions and shut down the hard drive.

In such cases, you may notice some stuttering in the device's performance, such as a movie or song pausing during playback. If you're using your Mac to record audio or video, you may also see a pause in the recording. But these effects are not limited to multimedia apps. If the SMS is activated, it can cause interruptions in other applications as well.

It's a good idea to know how to manage your Mac's SMS, including how to turn it on and off, and how to check whether it's working.

How to Check SMS Status on a Mac

Apple doesn't provide a way to specifically monitor the Sudden Motion Sensor system, but you can use Terminal to delve into the internal workings of any Mac.

  1. Launch Terminal, located in Applications > Utilities.

  2. Enter the following into the command line prompt:

    sudo pmset -g
  3. Press the enter or return key on your keyboard.

  4. Enter your administrator password and press enter or return.

  5. Terminal displays the current settings of the Power Management (the "pm" in pmset) system, which includes the SMS settings. Locate the sms item and compare the value to the list below to learn its meaning:

    sms – 0: Sudden Motion Sensor is disabled.

    sms – 1: Sensor is turned on.

    No sms entry: Your Mac is not equipped with an SMS system.

    SMS reading in Terminal

How to Enable the SMS System on a Mac

If you're using a Mac that's equipped with a hard drive, it's a good idea to have the SMS system turned on. A few exceptions are noted above, but in general, if your Mac has a hard drive, you're better off with the system enabled.

  1. Launch Terminal, located in Applications > Utilities.

  2. Enter the following into the command prompt:

    sudo pmset -a sms 1
  3. Press the enter or return key on your keyboard.

  4. If you're asked for your admin password, enter the password and press enter or return.

  5. The command to enable the SMS system doesn't provide any feedback about whether it was successful. You just see the Terminal prompt reappear.

    If you want assurance that the command was accepted, you can use the "How to Check the SMS Status on a Mac" method outlined above.

How to Disable the SMS System on a Mac

If your Mac is only equipped with an SSD, there is no advantage to attempting to park the drive's heads, because there are no drive heads in an SSD. In fact, there are no moving parts at all.

The SMS system is mostly a hindrance to Macs that have only an SSD installed. In addition to attempting to park the SSD's nonexistent heads, your Mac also suspends any writes or reads to the SSD while the SMS system is engaged. Since the SSD has no moving parts, there is no reason to shut it down because of a bit of motion.

  1. Launch Terminal, located in Applications > Utilities.


  2. Enter the following into the command prompt:

    sudo pmset -a sms 0
  3. Press the enter or return key.

  4. If you're asked for your admin password, enter the password and press enter or return.

    If you would like to ensure that the SMS is turned off, use the "How to Check the SMS Status on a Mac" method outlined above.

The SMS system is also used by a few apps that make use of the accelerometer. Most of these apps are games that use the SMS to add a tilt feature to the gaming experience. You can also find some interesting scientific uses for the accelerometer, such as an app that turns your Mac into a seismograph.

If the SMS doesn't seem to be working, your Mac's SMC may need to be reset.