MacworldHard disk drives (HDDs) work by spinning their internal platters. Only while spinning can data be read or written. Its been decades since they gained the ability to remain online butnotspinning 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 upand theyre 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 saysmacOS 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.A laptop Mac offers four options for hard drive sleep.FoundryA desktop Mac lets you enable or disable hard drive spin-down.FoundrySome 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 0The0means never, but you can set a higher value in minutes, like30to mean 30 minutes.You cant 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.Ask Mac 911Weve compiled a list of the questions we get asked most frequently, along with answers and links to columns:read our super FAQto see if your question is covered. If not, were always looking for new problems to solve! Email yours tomac911@macworld.com, including screen captures as appropriate and whether you want your full name used. Not every question will be answered; we dont reply to emails, and we cannot provide direct troubleshooting advice.