I have an ipod classic and I can not put the muisc on my ipod classic from an external hard drive

I have a PC, and all my music is on the external hard drive. My ipod will not sync. At the present time I have nothing on my ipod classic. I want to put the music on my ipod and keep the music on the external hard drive. When I open itunes I see all my music, and I can play the music. The music will not synch with my ipod. Any suggestions? Thanks for the help.