Thanks for your response!!
I connected my external hard drive to my computer and followed the steps that were listed in the apple support article under the "copy iTunes folder to your external drive" section. I don't have a Mac computer, but I followed the Windows steps. After the music had finished copying over to the external drive, I closed out the programs and shut off my computer. The next day when I went to play a song in iTunes, I got the error message for the majority (not all) of my music.
Here is the link for the instructions I followed:
Now in the past, I've had this issue with music that has been given to me by say a friend on a USB flash drive. I always thought that was the issue, but now it's giving me the same message for music that I bought the CDs and copied them directly onto my machine. I'm with you... I didn't think copying the library should do anything to my library, but I think my problem might lie somewhere in the steps listed for moving the file location before copying the library. As I open iTunes from my desktop shortcut, and look under "preferences" and in the "iTunes media folder location", I'm wondering if I need to do something with the "Reset" button to change what I had done from the Apple Instructions?
Also, as I connect my external drive up, it says it has all of my music (including all of the album artwork, iTunes library extras, etc.). I am able to open it up, but when I click on "iTunes library", it does the exact same thing as if I clicked my iTunes desktop icon. The music is not there since it is not the "original file".
Again, thanks for taking the time to help me out!