As you're backing up to an external drive, you *DON'T* want to clone your "C" drive - you want to create an "image" of it which can then be restored to your existing "C" drive or to a new hard drive in the event of problems.
When you create a backup "image", this will put one or more .TIB files in your external drive. These are the backup that you subsequently restore. You then still have the remaining space in the external drive for other file storage. Cloning would create an *EXACT* copy of your "C" drive, which could create problems if you wanted to use the external drive for storing other files.
When you restore the .TIB backup files, the resulting "C" drive is bootable.