I know the dedicated USB hdd's go to sleep but i was cheap when I set up my HDD.
My set up: 3.5" Sata HDD and ebay enclosure.
My situation: In the beginning I thought it wasnt too hard to switch the HDD on and off... Then I got lazy and decided I could make a relay do this for me using the other usb port to power the relay.

So here is my journey...

Required pieces:
Soldering Iron
Relay (I used OJE-SH-105DM)
Diode, I used a 1A 50V diode
3 wire extension cord or in this case mine took a standard computer power cord.
USB cable

Here are the supplies:

Now to start with find a old usb cable.. I used one from a old phone. Cut the end off and seperate the red and black wires from the other two. Cut off the other two.

Next drill a hole the size of your 120V power line in your project box. Make sure that you do it on the edge so that the top of the box completes the circle being that we wont be cutting all the wires.

Next drill a hole the size of your USB cable and slide the cable through.

Take your new relay and find which side is the coil side. The coil side will some resistance whereas the switch side will not(unless switched on)

Now take your diode and solder it on to the coil(it doesnt matter which side) **I accidently soldered it backwards so make sure to reverse it**

Once you have done that you need to solder the Red wire to the diode and the black wire to the other coil terminal. It should look something like this.

Now take your power cord and strip off about 1.5 inches of insulation in the area you want the relay box to be, make sure not to cut into the wires underneath. Now cut the white wire and strip the ends like this.

Now solder the ends onto the remaining two terminals on the relay, it doesnt matter which one goes to what.

You can now test it on a usb port and make sure the relay is functioning, if it is not check to make sure your diode is the correct way unlike in the picture.
Once it is working go ahead and neatly and delicately stuff it in your project box.

Put the lid on and your done!