My problem is that I want to power a hard drive off one of those 12v5v power bricks, but I need to be able to shut the hard drive on and off from another device's (separate power supply) usb port. (when the device is on it outputs 5v and when off it outputs nothing) Also the system could run for up to 4-5 hours continuously. (the hard drive current ratings are 12v=.55A 5V=0.70A)

 

It might just be me but I don't really understand what you are asking. Can you rephrase your question/problem please?

 

My problem is that I want to power a hard drive off one of those 12v5v power bricks, but I need to be able to shut the hard drive on and off from another device's (separate power supply) usb port. (when the device is on it outputs 5v and when off it outputs nothing) Also the system could run for up to 4-5 hours continuously. (the hard drive current ratings are 12v=.55A 5V=0.70A)

Run the power for the hard drive through normally open relay contacts with the coil of the relay powered by the 5V from the USB of "another device." Just be sure that the relay coil is 5V and that the current required is within the capability of the USB port.

Maybe this one? http://www.allelectronics.com/make-a-store/item/RLY-532/5-VDC-DPDT-MINI-SIGNAL-RELAY-2A/1.html
The coil is 5V and 178Ω, so the current should be only 28mA. The contacts are rated at 2A up to 30V.