Raspberry pi, 3.5" hard drives

Hi all,

I recently put together a NAS based on a Raspberry pi (model 2B) with 2 hard drives. It currently lives in a shoebox and works pretty well.
For those interested here is some rough schematics of the setup. The data connection is done by some USB to SATA adapters (ebay link).

There are still some some issues with power management. Hard disks keep spinning when the raspberry pi is shut down.
I am considering cutting the HDD 12V via a relay activated by a GPIO pin.

Do you have a similar setup and success with 3.5 HDD?

Thank you both for your replies.
Like you said, power consumption is hardly significant and is not really a concern.

Even spun down, the hard drives still make a hum, particularly noticeable at night.
Edit: unmounting and then remounting harddrives solved the problem. 100% silent now .

This will be used for backups on weekly basis, I feel it would be safer to have it disconnected from the mains when not in use. Doing that cuts the power to the disk when they are spinning full speed (raspberry pi turned off). I can also hear the heads parking in a not-so-reassuring noise...