Cannot boot/install Ubuntu from thumb or DVD

Hi.

I having trouble booting to Ubuntu (and other Linux installs) from both a Thumb Drive install, and a USB Superdrive.

Note. My hardware is a 2011 MBP, with 120GB SSD and 500GB HDD - Note* Superdrive was replaced.

I understand that Macs have trouble booting from a USB DVD drive - hence the troubles. But I cannot for the life of me understand why none of my attempts at a Thumbdrive install fail.

I have installed ReFit and have partinioned free space on my system drive.

When I restart and the ReFit menu appears. I select either
Boot EFI\boot\bootx64.efi from or
Boot EFI\boot\grubx64.efi from

Both options take me through to a GNU Grub screen, where I have the option to -
Start Ubuntu
OEM Install or
Check disk for defects.

When I select Start Ubuntu, the screen goes blank. I have no power light on the MBP, however the thumb drive is lit.. But it just sits there forever, before I reset.