Why do exclamation marks show up on your itunes library?

This happens to my iTunes because I have an external drive attached to my computer. The external drive backs up my data because I have had my C drive almost crash before. Sometimes when I purchase a song, the program saves the file under the K drive (this is the external drive on my computer). Once you find the song, and open it, it will no longer be "!" You will need to do some file search on the drive for the "itunes music" folder. The file will have a name of "download". Once you get the hang of finding it, you can correct this little problem.