Is your Hdd a very old one?Mine spin down on their own...not sure if it's due to hdparm enabled by default on LE but don't think so...most modern hdd's should do so..
AFAIK, only 2.5" USB HDD enclosures that have this command in their conntroller can spin-down HDD on Android. I think that there is no way Android OS can do that as Linux/Windows can. But this is another topic...