I replaced the path of the original file on the mac with the path of the copy of the same file on the NAS/USB by using the complete path as well as just using the file name. I used text editor (TextEdit) and tried to adapt the file location to the location of the copy on the NAS / USB Stick. I learned, by exporting M3U´s the path of the file location of the on my Mac is still in the file. I now did copies of the files on my NAS for listining via a Denon Receiver or Pioneer network play and on an USB-Stick for listening in my car. My iTunes library is on my Mac and thus files are located there.