when i drag books to my kobo i always get an error message telling me that im not allowed to copy the book. But i own that book.

when i drag books to my kobo i always get an error message telling me that im not allowed to copy the book. But i own that book. I'm running Yosemite on my brand new Imac. I've been with the Apple support support for quite a long time but I can't find a solution