How do you boot from backup drive?
 

This seems to be so basic that I cannot find an answer on the Mac site or here. Maybe a year ago I used SD to backup my internal drive which it has done and continues to update, but I've never tested it to see if my internal fails what I can do. It's simple, right?

System Preferences/Startup Disk/

Select the bootable drive you want your system to boot from. Close window and restart.

It should boot your machine from the disk you selected above.