[SOLVED] getting install onto external HDD on a mac

Hi, I'm trying to install ubuntu on an old PC laptop, but the CD drive is dead. I have an external drive that I thought it would be simple to copy the installer across using my mac and boot from that. For some reason osx doesn't like the install image and the built in disk utility won't copy it across. I tried following a few different sets of instructions online such as this one but with no luck. Converting the .iso into a .dmg worked but using the dd command (which I admit I don't fully understand) come up with a 'not supported' error...
any ideas?

Thanks,

Dan