There are many reasons for wanting to boot from an external drive. If you want to keep files in sync between desktop and laptop, expand your storage or have a bootable backup of your system, there is a hidden feature in Disk Utility that makes it easy.
Generally, booting from an external drive will be slower, even with new Thunderbolt and USB-C drives. These are still much slower than solid state drives (SSD) found in most new Macs. Then, even though this is not recommended for everyday use, it is still feasible.
Directly from Disk Utility, no third party apps required
Launch Disk Utility from Spotlight (Command + Space) or from the Utilities folder in your applications. You will be greeted with a list of all your volumes, including its internal hard drive (probably called OS X or Macintosh HD) and your external hard drive.
This is where we get to that hidden feature we mentioned..
The button “Restore” in Disk Utility you will copy the files from a backup to your main drive. It is designed to be used from recovery mode to restore your hard drive after a failure.
But, if you choose your external drive as the restore target, you can reverse that action and copy files from your main drive to backup. Select your external drive in the sidebar, click on “Restore” on the menu, and then select your head unit as the option “Restore from”. You can also select an ISO image, but that doesn't have much use here.
Click on “Restore” and Disk Utility will start the copy procedure. This can take a long time, depending on the speed of your external drive and its connection to your Mac, so it is better to have a fast hard drive with Thunderbolt connections, USB-C o USB 3.0.
And that is! When Disk Utility finishes, you can shut down your Mac and hold the Option key when you restart it. This opens the boot switch and enables you to boot from the external hard drive. You can use your Mac as usual, but note that it is separate from the installation on your main internal hard drive. Any settings you change or files you save there will not be reflected in your main install.
You can do the same procedure in reverse if you need to re-copy the files, or restore the backup if your computer decides to fail.
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