CF (Compact Flash) uses the IDE interface, identical to internal IDE hard drives. I have seen mods of equipment using CF to work with HDD's, and this is also how the CF based internal hard drive adapters work (you can buy one for very cheap to be plugged into one of your HDD slots, to speed up disk access. It is essentially just wiring, no electronics required). Given that, I can imagine modding a CF-based camera to work with external drive.
SD is a different story, as it implements a completely different interface. I think there are SD to CF adapters that will let you use a SD card in a CF device, but I'm skeptical that the reverse conversion exists. So, with a SD based camera, your only solution seems to be shooting tethered with EOS Utility.