/Applications/Install OS X El Capitan.app/Contents/SharedSupport/InstallESD.dmg Copy this file to your Desktop.
On a Mac with the El Capitan installer app: macosxelcapitan10111imageiso work
Open Terminal and run:
Double-click InstallESD.dmg to mount it. It will appear as a volume named “OS X Install ESD”. /Applications/Install OS X El Capitan