Will a hard drive use less power if it is unmounted?

I recently did the Optibay mod on my 13" Macbook Pro, the stock magnetic drives now lives where the optical drive used to be. And an SSD has taken its place.

It's all good, but power consumption seems to have shot up as a consequence. I have tried various things but can't seem to get over this. I was contemplating unmounting the drive at boot time and manually mounting it when I need to use it (its a data/backup drive so only used occasionally) but I'm not sure this will work.

Does anyone have any knowledge of this?

An unmounted drive will not react to system sync(2) calls. Consequently it will remain idle (spun down) and not contribute much to power consumption. To alter how soon the drive spins down, you may use the pmset(1) utility as described to some extent in this answer