The Spin-Up Delay
macOS includes aggressive power management features designed to extend battery life and reduce electricity consumption. One of these features affects mechanical hard drives (HDDs) connected either internally (on older iMacs) or externally via USB. If macOS detects that the drive hasn’t been actively read from or written to for a few minutes, it will automatically spin the drive platters down and put the disk to sleep. The problem occurs when you suddenly need a file. You click the drive in Finder, and your entire Mac freezes for 5 to 10 seconds, waiting for the physical disks to spin back up to operating speed before it can display the contents.
Keeping the Disks Awake
To eliminate this frustrating lag and stop macOS from automatically putting your connected hard disks to sleep during periods of inactivity, you must modify the system’s energy settings.
Click the Apple logo in the top-left corner of the screen and select System Settings (or System Preferences). If you are on a desktop Mac (like a Mac mini or iMac), click on Energy Saver. If you are on a MacBook, click on Battery, then click the Options… button at the bottom right. Look for a checkbox specifically labelled Put hard disks to sleep when possible. By default, this box is checked. Click the checkbox to remove the checkmark. From now on, as long as your Mac is awake, any connected mechanical hard drives will remain spinning continuously, ensuring instant access to your files without the spin-up delay.