Originally Posted by cybmole
UPDATE - it is NOT user error its a bug.
I did 3 tests
1. create a folder called test - add a single epub book to that folder. use add books - OK
2. ditto - but use the option: add books assume all books are the same book option - OK
3. repeat with 2 book formats in the test folder - an epub and a pdf of the same book. this time they both vanished from test folder after the add books options completed
I can confirm that step 3 above does cause the source files to be moved instead of copied to the calibre library directory.