Apple updated its Apple Worldwide Developer Relations Intermediate Certificate in February 2016 after they expired. This has the expected side affect that all USB installer created for Mac OS X (El Capitan, Yosemite, Mavericks, Mountain Lion, and Lion) will now longer work. We need to create a new installer. This also has the added benefit of having post of the security updates already installed.
First thing to do is format a USB disk which should be 8GB or larger. Using the Disk Utility, Format the drive as OS X Extended (Journaled) and label the drive: Installer.
After that, download the latest OS X El Capitan from the App store but don’t run it.
Once it has downloaded, type or paste this command into the terminal:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Installer --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
The system will ask for your password, once you have entered the password, Mac OS will create the installer disk. This can take up to 30minutes.
You should see the following:
Erasing Disk: 0%… 10%… 20%… 30%…100%…
Copying installer files to disk…
Making disk bootable…
Copying boot files…
Once it finally done, remove and insert the USB drive and reboot the Mac. Hold down the Option Key which will then launch the installer.