There are many reasons to boot from an external drive. If you want files to stay in sync between a desktop and a laptop, expand your storage, or have a bootable backup of your system, Hard Disk Utility has a hidden feature that makes it easy.
Normally booted from external The drive is slower, even on new Thunderbolt and USB-C drives. These are still much slower than the solid state drives (SSDs) of most new Macs. Although not recommended for daily use, it is still possible.
Directly from Disk Utility, no third-party apps required
Launch Spotlight's Disk Utility (Command + Spacebar) or the Utilities folder in your applications. You'll be greeted with a list of all your volumes, including your internal hard drive (probably called OS X or Macintosh HD) and your external hard drive.
Here we come to this hidden function we mentioned.
The Restore button in the Disk Utility copies the files from a backup to the main drive. It should be used from recovery mode to recover your hard disk after a failure.
However, if you select your external drive as the restore destination, you can reverse this action and copy files from your main drive to the backup. In the sidebar, select your external drive, click "Restore" on the menu, and then select the main drive as the "Restore From" option. You can also choose an ISO image, which is not useful here.
Click Restore. Disk Utility starts copying. This can take a long time, depending on the speed of your external drive and its connection to your Mac. Therefore, it is recommended to use a fast hard drive with Thunderbolt, USB-C or USB 3.0 ports.
And that's it! When the Disk Utility is complete, you can shut down your Mac and hold down the Option key when you restart it. This will bring up the boot switcher and allow you to boot from the external hard drive. You can use your Mac normally, but keep in mind 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 primary installation.
You can perform the same operation in reverse order if you want to rerun the files or restore the backup if your computer damages the data