Manual Mode Problem Transferring Songs To iPod

Hi,
I just bought a second iPod (used 4th generation). I have a Library on an external hard drive. I am trying to transfer some of the Library to the new iPod. While the new iPod is recognized in iTunes, when I drag something over to copy onto the iPod, I just get the circle with the line through it, not allowing me to copy the contents to the iPod. What gives?