You can not drop files
direct into the Library folder.
To add formats, you can drop books into the
Details Pane for that book,
or into the
Formats section of the Metadata editor.
NOTE: doing this way allows you to drop ANY book there
, there is no 'checking' done.
Add Books or copy to Library (from another Calibre Library) reads metadata and updates or creates (controlled by adding books preferences) a new record.
Use of a Networked or Cloud drive can scramble entries in unpredictable ways, especially during 'updating the book' activities. ALWAYS PAUSE sync when running Calibre