Official comment
Hi Hanna,
If the track can't load, that means the file itself has moved. If you're on a Windows machine, consider giving the source drive a fixed drive letter so it doesn't change depending on what other drives you have connected to your computer.
If you're using iTunes at all, be sure the preference for allowing iTunes to manage your music (move the files when you make changes to anything) is UNCHECKED.
You can relocate the music using this process.