Cloning an iso file to a mass storage device makes a boot drive, provided it is a hybrid iso file, post-processed with isohybrid.
If the program isohybrid is not found, you can install it with the following program packages
in 14.04 'Trusty' the package containing isohybrid is syslinux
in 15.04 'Vivid' and newer versions, isohybrid is in the package syslinux-utils
sudo apt-get install syslinux # or sudo apt-get install syslinux-utils
# run this command only to keep original iso file cp -p filename.iso filename-hybrid.iso # the actual command isohybrid filename-hybrid.iso
See the manual file for more details,
Wait, don't do this automatically and at once! Isohybrid should be run only once, and many iso files are already treated before uploading to the internet. The Ubuntu iso files are already treated with isohybrid. Use isohybrid only, if the USB drive does not boot after cloning or installing live-only using mkusb.