How do I get my Mac to boot from an Ubuntu USB key?
To create an Ubuntu 12.04 LTS USB stick in Mac OS X Lion, how should be the stick formatted initially?
Welcome to Ask Ubuntu, lsemac!
You don't need to format the USB before recording the USB, just make an USB image and burn the IMG using Disk Utility. The drive will be formatted automatically.
If you want to install Ubuntu in a USB drive:
As it seems too difficult, I'm going to make a Python script which can simplify the process :)