Huge emphasis on WHOLE
The containing folder is the Library NAME (as seen on the Menus) and can be different (you will need to select the new Library at start, but that is all that is needed).
The CONTENTS must be EXACT (and
not mixed with other folder contents)
When I make a Backup copy, I simply rename the folder with the Date.
If I need to
recover 
a book:
I just attach the backup media: Switch Calibre to (Library) 20141008main: (select the book) Copy to Library 'Main': Switch back to Main and disconnect the backup (store away safely) and I am good to go.
The only time Calibre was NOT involved was the initial backup. I use the Xcopy command as it is faster than using Calibre to Copy a Full Library (it can do that just fine if you got the time.)