So I bought a 3tb usb 3 Seagate backup plus drive to use with time machine on my 27 inch 2012 imac. It works great, except that it never goes to sleep and stops spinning. My mac is set to "turn off disk drives when not in use". It turns off the screen and the internal fusion drive, but the external seagate never stops spinning... even when the imac has been out for hours. Anyone have this same drive?