If you want to add an ebook format file to a book, remembering there can only be one of each kind (EPUB, PDF, AZW etc), rather then opening MDE doing the drag drop and closing the MDE, I find it easier to drag drop the additional format file onto the Book details panel.
To add files to the Data folder, I open the folder (calibre will fire up Windows File manager) and then I Drag-n-Drop or Cut/Copy Paste files into it… more often than not from the Desktop. As I see it, the management of the content of the Data folders is my responsibility.
BR
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