https://www.howtogeek.com/399058/how-to-make-a-bootable-clone-of-your-macs-hard-drive/
There are a lot of reasons for wanting to boot from an external drive. If you’d like to keep files synced between a desktop and laptop, expand your storage, or have a bootable backup copy of your system, there’s 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. Those are still much slower than the solid state drives (SSDs) found in most new Macs. So while this isn’t recommended for daily use, it’s still possible.https://imasdk.googleapis.com/js/core/bridge3.496.0_en.html#goog_1199320800 seconds of 30 secondsVolume 0%
Straight from Disk Utility, No Third Party Apps Required
Fire up Disk Utility from Spotlight (Command+Space) or the Utilities folder in your applications. You’ll be greeted with a list of all of your volumes, including your internal hard drive (probably called OS X or Macintosh HD) and your external hard drive.
Here’s where we get to that hidden feature we mentioned.
The “Restore” button in Disk Utility will copy the files from a backup to your main drive. It’s intended to be used from Recovery mode to restore your hard drive after a failure.ADVERTISEMENT
But, if you choose your external drive as the restore target, you can flip that action around and copy files from your main drive to the backup. Select your external drive in the sidebar, click “Restore” in the menu, and then select your main drive as the “Restore From” option. You can also choose an ISO image, but that doesn’t have much use here.
Click “Restore,” and Disk Utility will start the copying process. This can take fairly long, depending on the speed of your external drive and its connection to your Mac, so it’s best to have a fast hard drive with Thunderbolt, USB-C, or USB 3.0 connections.
And that’s it! When Disk Utility is done, you can shut down your Mac and hold down Option when booting it back up. This brings up the boot switcher and lets you boot from the external hard drive. You can use your Mac as normal, but keep in mind that it’s separate from the installation on your main internal hard drive. Any settings you change or files you save there won’t be reflected on your primary installation.
You can do the same process in reverse if you need to copy the files back over, or restore the backup should your computer decide to break.