The Entire Library folder IS the Library.
so DO NOT copy (mix in) any other files or folders.
that simple
If your Library folder was My Books,
The (Backup) the entire Folder: My Books
to restore the Library.
Copy the entire folder back to your HD, then start Calibre and
Use the Switch Library option on the Library Icon, to set Calibre to use that Library.
Re-Format HD many times? It has been years since I had to do that (dying drive). I have cloned drives (bigger
) that gave warning signs (look at the event viewer logs)