Rekordbox not exporting from External Hard Drive to USB

Hi,

Most of my music has been imported to my Rekordbox library from an external Hard Drive, however there are some mp3 files that are on my internal HD on my mac. When I create playlists and try to export to a USB Pendrive, only the track from my internal memory get exported. The error message says 'One or more files were not exported'. Any ideas why I can't export the files from my external HD?

Cheers,

Boris