https://support.apple.com/en-au/HT201372

Create a bootable installer for OS X

With OS X El Capitan, Yosemite, or Mavericks, you can use a USB flash drive or other removable media as a startup disk from which to install OS X.


These advanced steps are intended primarly for system administrators and others who are familiar with the command line.

Use the 'createinstallmedia' command in Terminal

  1. Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
  2. Mount your USB flash drive or other volume. You could also use a secondary internal partition.
  3. Open the Terminal app, which is in the Utilities folder of your Applications folder.
  4. Use the createinstallmedia command in Terminal to create the bootable installer. Examples of this command are in the next section. For detailed usage instructions, make sure that the appropriate Install OS X app is in your Applications folder, then enter one of the following paths in Terminal:

Path for El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

Path for Yosemite:

/Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia

Path for Mavericks: 

/Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia

Examples

This is the basic syntax of the command. Replace volumepath with the path to your USB flash drive or other volume, and replace installerpath with the path to the Install OS X app. 

createinstallmedia --volume volumepath --applicationpath installerpath

The following examples assume that the OS X installer is in your Applications folder and the name of your USB flash drive or other volume is MyVolume:

Example for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Example for Yosemite:

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

Example for Mavericks:

sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app