HDD spin down is a function of the enclosure that ensures efficient operation and reduces power consumption. When the host system is turned off, in sleep mode, or when the enclosure is safely removed from the host system, the enclosure will direct the hard drive to spin down; in turn, minimizing power consumption. The hard disk will spin up automatically once the host system is turned back on or woken up from sleep mode. When you touch the enclosure, you will notice that the spindle of the hard drive will automatically turn on. The beneficial function will increase the life of the device and reduce excess power consumption. All of our SATA HDD enclosures, with the exception of ST-M10 (it has been phased out), support the HDD spin down function. Please note that as long as the AC adapter is plugged in, the LED will remain lit even when the hard disk have spun down.