I haven't had to do this in a long while, and when I used to do it, I would use WinAmp. I seem to remember a simple drag-and-drop interface working between WinAmp and Windows Explorer. I can get it to work if I drag files to WinAmp's Local Media, then copy them to the iPod, but that's two steps, instead of one.
I was hoping iTunes could browse my hard drives and let me add files straight to the iPod, but I can't get past this screen, which obviously wants me to buy my music from the iTunes store.
The only pathways I can find involve adding media to the "Library," which is impractical--I have terabytes of music. Isn't iTunes capable of browsing a hard drive?