You can't do that without some sort of device in between your camera and the hard drive. USB is a "master : slave" protocal - one device is the master and the other the slave. Slaves cannot communicate with other slaves. And guess what - the USB port on a camera is a "slave" port as is the USB port on a portable hard drive.
That being said, one of the many more powerful micro-controller cards available (one with USB master hardware) could probably be setup to do the job via ptp.
