The interface has become possible via the Camera Connection Kit which was originally intended for interfacing with external camera SD Cards. Maxwell C. Shay hacked the way to make it possible to connect external HDD with iPad with a little effort.
Shay uses a split USB cord to provide the sufficient power to turn the hard drive ON while connecting one end to the Camera Connection kit and the other to a power source, in this case it’s a Macbook. If external disk is powered from an external source (say adaptor), connecting it to laptop is redundant.
Pre-requisite: Jailbreak iPad with Spirit
How to Mount iPad external Hard Drive
Video with instructions is available below, if you prefer Text instructions, navigate here.