Rekordbox - Files keep Go missing and need to be reconnected mannually

Hi folks, I bought a new May M2 and put my music files on Documents > Dj > Music > According Folder as mp3 files. When I go back to Rekordbox, some files in my playlists are visible, but as soon as I want to export them on the stick, I get an error message. Files seem to keep go missing and I have to reconnect them manually afterwards. I don't really understand WHY that thappens in the first place, since I do not move the original files and also I am curious if you know a solution how to fix this? Let me know! Thanks a lot, cheers, Cee