To be able to use an external HDD with the Pi you need external power. With this hack you'll be able to power both the Pi and the HDD with the USB hub.
Update: powering old phone used for tethered internet access.
Step 1: Gather Parts
USB hub: http://pages.ebay.com/link/?nav=item.view&id=151854967357&alt=web
Micro USB breakout: http://www.banggood.com/50Pcs-CJMCU-Micro-USB-Interface-Board-Power-Switch-Interface-p-997115.html
Step 2: Open and Detach Port
To open the USB hub you need to "rip out" the end cap without a USB port in it.
Once loose you may gently remove the plastic casing containing the circuit board from the metal casing. You might need to help it out by bending the ports downwards to pass the openings in the metal.
Carefully cut away the plastic covering the ports.
Desolder one of the ports.
Step 3: Solder the Micro USB Breakout to the Hub
First you need to solder two wires to the vcc and gnd pads on the breakout. Leave the data pins so it's a power port only.
Solder the wires to the vcc and gnd pads on the hub. I had to cross the cables to align the pads. Glue the breakout to the plastic casing and reassemble.
Step 4: Usage
The micro USB port at the end cap goes to one of the usb ports on the pi. The other one goes to the wall wart. One of the USB A ports go to the micro USB power port on the pi. The last two available ports go to your HDD's or other devices.