I did back up Sierra onto my external drive. I am assuming that is similar to a clone???
How did you make the backup? A clone is an exact copy of the entire volume. You can boot from the clone and use it as if it were an internal drive. Did you use a cloning app?
Carbon Copy Cloner and
SuperDuper are the ones that I am familiar with. Both work very well but I prefer CCC because it has some features that I like.
If your backup was made by drag-and-drop, you copied the files but not the essential system. You may be able to access those files, but you won't be able to boot into the backup. For that, you absolutely need a cloning app. Disk Utility can do it, but I have never used DU for that purpose and I can't speak for how well it works.
