Disk Spindown when idle - RMerl/asuswrt-merlin.ng GitHub Wiki

Jeff Gibbons's sd-idle-2.6 has been added to the firmware, allowing you to configure a timeout value (in seconds) on the Administration -> System page. Plugged hard drives will stop spinning after being inactive for that specified period of time. Note that services like Download Master might be generating background disk activity, preventing it from idling. Also, not every hard disk or USB enclosure will support this.