This may be obvious but I have attempted to boot from a bootable USB on my MacBook Pro (2011, running Lion 10.7.5), the USB does boot but fails to finish the full boot process (it is a Ubuntu derived bootable linux USB).
In terms of what happens during boot, well it goes to the grub options and after selecting, it will start to boot, get to a certain point and then "stop" and hang with nothing on the screen.
I have used this same USB to boot into other, newer MacBooks with success.
Has anyone else run into this issue?
NOTE: The usb is formatted FAT32