Make sure you are using the firmware v2.6.3 or later, go to the "Disk Utility" menu in the Maintenance area and enter a time (e.g. 10 minutes) for the disk spin down function.
Note: The "disk spin down" function might not work properly for some of the WD hard drives due to lack of the E3h command. Also, from time to time, the system will access the main drive, so at least one of the drives will keep spinning and you might see disk activity even when the device is not in use. This is normal and to be expected.
