I wanted to upgrade my 10.04 PC to 12.04. Towards the end of the upgrade, I made the foolish mistake of thinking "Hibernate" on the system menu meant "Sleep", when it reality it seemed to shut down the computer. When the computer restarted it exploded with the weirdest errors, cumulating in "Plymouth main process (56) killed by SEGV signal".

So since I didn't have any files I cared about on the computer, I decided to go ahead and clean install 12.04. I used unetbootin on my Macbook Air (no CD drive) to install 12.04 via a downloaded DMG, and tried to boot up the computer to start the installation. I ended up with a "Missing operating system" error.

What am I doing wrong? I've read that I need to make the USB drive "bootable", but doesn't unetbootin take care of that? It is a Samsung USB drive, but I've reformatted and wiped it so there's no U3 partition.