When you receive the error message, there should be an option to navigate to the location of the .wav data. If you know where the files are, please navigate to them. If you open Studio One and go to Studio One>Options>Locations>User Data, you will see a checkbox that you can check to have the program prompt you to copy external files when saving a song. If you check this box, all media files will be copied to the "Media" folder inside of the Song folder that you are working on and you will not have this problem.
*If all else fails, please go into Documents>Studio One>Songs and delete the folder named "cache." This will be re-build when you re-open the song.
