With everything prepped, it's time to use the OS X Recovery Disk Assistant to create the Recovery HD.
- Make sure your external drive is attached to your Mac, and that it shows as mounted on the desktop or in a Finder window.
- Mount the OS X Recovery Disk Assistant disk image you downloaded from the Apple web site by double-clicking its icon. (If you haven't yet downloaded the application, you can find a link to it on Page 2 of this guide). It will probably be in your Downloads directory; look for a file called RecoveryDiskAssistant.dmg.
- Open the OS X Recovery Disk Assistant volume you just mounted, and launch the Recovery Disk Assistant application.
- Because the application was downloaded from the web, you will be asked if you really want to open this application. Click Open.
- The OS X Recovery Disk Assistant license will display. Click the Agree button to continue.
- The OS X Recovery Disk Assistant will display all external volumes connected to your Mac. Click the volume you wish to use as the destination for the recovery volume. Click Continue to start the creation process.
- You will need to provide an administrator account password. Supply the requested information, and click OK.
- The Recovery Disk Assistant will display the progress of the disk creation.
- Once the recovery volume is created, click the Quit button.
That's it; you now have a recovery volume on your external drive.
A few things to note: The recovery volume is hidden; you won't be able to see it mounted on your Mac's desktop. Additionally, the default installation of Disk Utility won't be able to show you the hidden recovery volume. There is, however, a simple way to add the ability to view hidden volumes to Disk Utility by enabling its debug menu.
You should test your new recovery volume to confirm that it's working. You can do this by restarting your Mac while holding down the option key. You should see your new Recovery HD as one of the startup options. Select the new Recovery HD and see if your Mac will successfully boot and display the recovery options. Once you're satisfied that the Recovery HD is working, you can restart your Mac normally.