If the 'data' folder doesn't exist when you open it, calibre will create it…before opening it.
You can add, delete and rename subfolders and files in the 'data' folder as you please… the things you shouldn't do are:
- rename the 'data' folder;
- move files from the book folder to data folder — use file manager copy to data folder, and then use calibre to remove from book folder;
- move files from the data folder to book folder — use calibre to add the file to book files, and file manager to delete the file from the data folder.
If the 'data' folder is no longer required, open the book folder and delete it.
If a 'data' folder is empty the 'has_extra_files" template function will return false.
BR