You can run this command: sudo pmset -a disksleep VALUE (replace Value with a number 1-10...1=1min, 10=10mins or longer if u need)
Also make sure nothing is accessing the drive....I have purchased paragons NTFS for OSX and that wouldn't let my drive sleep as it continuously did a chache file check on my bootcamp partition....the company said they are working on the known bug and should release a update soon, so for now I have disabled the driver unless needed....my drive sleeps within 1min and stays as sleep until I need it (only sometimes it spins up on its own randomly...when launching dashboard or safari....but goes back to sleep thats why I choose a smaller value)
If you have other things like ALIAS or applications accessing your HDD then it won't sleep. Like I have Vuze which downloads torrents to my HDD so when thats running obviously my drive doesn't sleep.....you will have to check into it depending on how you set up your applications/files etc.