I created a bootable USB stick with Ubuntu 12.04 on it using these instructions.
Then I tried booting my MacBook Pro with the USB by holding alt during start-up. I chose the option
Try Ubuntu in the list that appeared.
I got a black screen for about 5 minutes, so I decided to abort the boot by holding the power button for a while. The laptop turned off, and after turning on and booting into OSX all was fine.
However, I think I am missing some hard drive space after this failed USB boot. How can I check if I am actually missing drivespace and how can I restore this (remove the residual files left by the USB boot)?