Try hdparm - search here on the forum.
I use this setting:
hdparm -S 48 /dev/discs/disc0/disc
-S 48 means 4 min w/o operating hdd goes to sleep.
In any new activity it wakes up and spining.
Because -K switch doesnt operate (for making the setting permanent) , I run this in initial script on every startup.
Works fine with my Toshiba 2,5" hdd.