I have been trying to make a bootable Ubuntu live usb disk and none of the methods I found online worked for me. I followed the official instruction from here but the drive didn't even show up as an option when booting up. I also tried the unofficial method described here using a Ubuntu VM and this time I got the "Missing Operating System" error before the installation could begin. Installing rEFIt, using the latest 11.10 beta and the 64bit version didn't work. I don't have an optical drive in my 2011 MBP so I NEED to get this to work. Please help.