Save Your Mac's Battery - Spin Down Your Drive's Platters

Put Your Hard Drive to Sleep to Save Battery Life

Use Terminal to Spin Down Your Mac's Hard Drives
Screen shot courtesy of Coyote Moon, Inc.

I've been using my 15-inch MacBook Pro on the go more than usual recently, and in doing so, I've discovered that I have battery usage issues. There's nothing wrong with the battery; the problem is me. I've been amazed at how quickly I use up the battery power on my MacBook Pro.

There are a ton of ways to manage your portable Mac's battery performance, ranging from the obvious (put your Mac to sleep or shut it down when you're not using it) to the silly (switch to older versions of apps and OS X, the theory being that older apps don't have as many features, so they put less stress on the CPU).

Sorry, I'm not going to install MacWord, even if I could.

There are plenty of realistic ways to manage your Mac portable's battery life, and in this tip, we take a look at one method we often forget about.

Spinning Hard Drives Sap Battery Power

Although Apple offers SSDs (Solid State Drives)​ in many of its Mac portables, the old-fashioned hard drive is still the most common storage media. Hard drives have a lot going for them; they cost less per GB of data, and they can hold a great deal more data than any of the standard SSDs that are currently available.

But hard drives do have one major drawback for portable users: they use a lot of energy. In order to access data on a hard drive, its platters must be spinning; this means the drive's motor spends much of its time sucking up juice to keep the platters rotating at high speeds; usually 5,400 or 7,200 RPM.

OS X can put hard drives to sleep, essentially telling them to turn the motor off and let the platters spin down.

That saves a lot of energy, although it also means that when you want to access data on the hard drive, you'll have to wait for it to spin its platters back up to speed.

It would be nice if OS X gave you some options for when the platters would spin down, but the only built-in option in the Energy Saver preference pane is to "put the hard disk(s) to sleep when possible." What this option actually does is put a drive to sleep if there has been no access for 10 minutes.

That's too long a wait for my taste; somewhere between 3 and 7 minutes would provide better battery life.

Changing Disk Sleep Time

Changing how long your Mac waits before spinning down its hard drives is fairly simple; you just need to make a small change in the pmset utility, which OS X uses for power management. To make the change, we're going to use Terminal, the trusty application of choice that we use to modify many of OS X's default behaviors.

Pmset allows you to make changes for when your Mac is running on a battery or when it's running on AC power. We're only going to change the power management profile for when the Mac is running on batteries. We'll do that by using the "-b" flag in the pmset command. In this example, we will set the disksleep wait period to 7 minutes.

  1. Launch Terminal, located at /Applications/Utilities.
  2. Enter the following command at the Terminal prompt:
    sudo pmset -b disksleep 7
  3. Press enter or return.
  4. You will be asked for an administrator password. Type in the password and press enter or return. Your password won't display, so don't be alarmed when no text appears as you type the password.

That's all there is to it. When running on battery power, your Mac will wait for 7 minutes of inactivity before spinning down its hard drives.

You can change this setting as many times as you wish, so don't worry if you need to fine-tune the wait time to suit the way you use your Mac.

By the way, if you set the wait time to zero, the hard drives will never spin down.

Published: 2/24/2012

Updated: 8/27/2015