Change this to match the name of your USB stick. Where MyVolume is the name of your USB stick.
Sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia -volume /Volumes/MyVolume -applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app Create a bootable image from the El Capitan installer on a USB stick larger than 6GB.It will be downloaded to /Applications and be named Install OS X El Capitan.app It might complain that you already have it installed but download it anyway.