I have an existing portable hard drive and i want to be able to offload photos to it on go without a computer.
No, the external drive is a USB slave device, not a USB master device. It has to attach to something that has a USB host controller, such as your computer. As pointed out above, there are stand alone devices with card readers for the purpose you describe, but they have a host controller built in so that they can read memory cards. They are also much more expensive than a standard external hard drive, although the units without a display are less expensive than the units that also have a large LCD to view your pictures.
