I currently have Linux Mint 17, and I want to dual boot it with Ubuntu 14.04. I tried the steps on the website to boot it from a USB drive (https://help.ubuntu.com/community/Installation/FromUSBStickQuick) and I can't use Universal USB Installer since I already have Linux. It has to be run on Windows. I can't do it with Wine. I found an alternative called Unetbootin. It says I need extlinux. I ignored it, did everything I needed, rebooted the computer, starts up Mint. I try it again, but this time installing extlinux first. Do everything the same. Reboot. Starts up Mint. Gives me no option. I have made sure that the files are on my USB drive, and it doesn't work. What am I doing wrong?
Maybe you have been trying to run UNetbootin in Wine. However you can also install UNetbootin from the Software Manager, and it should run without saying it needs extlinux and you can make an Ubuntu 14.04 live USB with a >=2GB USB flash drive by following these instructions.