Is it possible to use an external hard drive to start a live Ubuntu session?

Is having an ISO file as the whole content of the first partition of the device enough?

Thank you for reading...!

EDIT upon reading the first comment to my original question:
If I remember correctly, I COULD run a live session of Oneiric Ocelot somehow. It was not from a CD because I failed to burn one, so it must have been from an ISO file. Still very very confused....