There are many reasons to want to boot from an external drive. If you want to keep file synchronization between a desktop computer and a laptop, expand your storage space, or have a bootable backup copy of your system, the Disk Utility hides a feature that makes it easier.
Generally, booting from an external drive will be slower even with the new Thunderbolt and USB-C drives. These are still much slower than the SSDs found in most new Macs. So, although this is not recommended for everyday use, it is still possible.
Directly from Disk Utility, no third-party application required
Launch Disk Utility from Spotlight (Command + Space) or the Utilities folder of your applications. You will receive a list of all your volumes, including your internal hard drive (probably called OS X or Macintosh HD) and your external hard drive.
We come here to the hidden feature that we mentioned.
The "Restore" button in Disk Utility will copy the files from a backup to your primary drive. It is planned to use the Recovery mode to restore your hard disk after a crash.
However, if you choose your external drive as the target of the restore, you can reverse this action and copy the files from your main drive to the backup. Select your external drive in the sidebar, click "Restore" in the menu, then select your primary drive as "Restore from" option. You can also choose an ISO image, but it does not help much here.
Click "Restore" to have the disk utility start the copy process. This can take a long time, depending on the speed of your external drive and its connection to your Mac. It is better to have a fast hard drive with Thunderbolt, USB-C or USB 3.0 connections.
And that's all! When the disk utility is finished, you can turn off your Mac and hold Option while saving. This opens the boot switch and allows you to boot from the external hard drive. You can use your Mac as usual, but keep in mind that it is separate from the installation on your main internal hard drive. The settings you change or the files you save will not be reflected in your primary installation.
You can perform the same process in reverse if you need to copy the files or restore the backup if your computer decides not to work properly.
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