How to stop a Mac’s hard drives from spinning down

Hard disk drives (HDDs) work by spinning their internal platters. Only while spinning can data be read or written. It’s been decades since they gained the ability to remain online but not spinning in a standby state. This reduces power and can also reduce wear and tear produced by heat output and mechanical usage.

macOS automatically sends a signal to internal HDDs (on older Macs) and external ones that tell them to spin down after a period during which there has been no access to data stored on them. The next time an HDD is needed, there is a slight delay while the drive spins up to speed. Some people have an intermittent but continual need for HDD usage, and they find that the constant short delays add up–and they’re rightly concerned about frequent spinning up and down causing its own set of wear-and-tear issues.

Apple provides a friendly way to control that behavior in System Settings via the “Put hard disks to sleep when possible” setting:

• On a Mac laptop, go to System Settings > Battery and click Options. You can choose between Always (the default) and three alternatives: Only on Battery, Only on Power Adapter, or Never. Only on Battery or Power Adapter puts a drive to sleep when possible only with that energy source, which may remain desirable. Never will do as it says—macOS never attempts to spin a drive down.
• On a Mac desktop, go to System Settings > Energy. There, you can enable or disable the setting, as desktop Macs require a power adapter.

Some HDDs and some installations of macOS may ignore that setting and continue to spin an HDD down after a short interval. If you find that unacceptable, you can open Applications > Utilities > Terminal and enter the following command followed by a Return; enter an administrative password when requested:

sudo pmset -a disksleep 0

The 0 means “never,” but you can set a higher value in minutes, like 30 to mean “30 minutes.”

You can’t issue a command line instruction to reset the HDD sleep duration to the default, which appears to be 10 minutes on an otherwise unchanged Mac desktop I tested on.

This Mac 911 article is in response to a question submitted by Macworld reader Howard.

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