USB booting :: how to make a usb-hdd boot with usb-zip

Hi,How do you make another distro (not dsl - ubuntu) boot from a portable hard drive boot as if it was a usb-zip device? I want to use a portable hdd with ubuntu linux (dsl is great but its just not as powerful as ubuntu) but it needs to emulate a floppy or a usb-zip drive to boot.

original here.