====== How to make a bootable USB Stick with El Captain ======
  - Download der OS X El Capitan installer into the Application folder via App Store. The installer will remove itself when you run the installer, but you can download the installer again.
  - Format an USB Stick (> 8Gig) with Extended (Journaled) HFS+ with the name "foobar" or something you like.
  - Go to the Terminal and type

<code>
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/foobar --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
</code>

How you have a bootable USB Stick.

You can choose the boot disk with system preferences or by holding der option key.