This is how to create a OS X El Capitan installer on a flash disk or external disk.
Download OS X El Capitan from the App Store. When it finishes downloading, quit the installer. The “Install OS X El Capitan” application should be in your /Applications folder. On a side note even if you do not create a bootable installer, it’s a good idea to get a backup of this installer before running it. After upgrading your system, the installer will remove itself.
Format a flash stick Mac OS Extended (Journaled) with GUID Partition Table
Name the disk “Untitled”. Simply because the command below references the disk by the “Untitled” name.
In Terminal, run:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia –volume /Volumes/Untitled –applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app –nointeraction