Do you intend to keep your machine active all night?
Where is this hdd located?
If you have a NAS you can program it to sleep, in which case the disks spin down and power usage is about 5W.
I also don't agree that there is a problem in shutting down the hdd.
What you need to do is spin the disk down and have the machine hibernate. I would suggest 1 hour idle time and teh disk can spin down (shut down).