I had the same problem with my iPod Video 5G. I was unable to copy any file from the iPod, not just music.
I fixed it by restoring my iPod with iPod Updater 2006-06-28 and reinstalling Rockbox. I use this older version of the iPod Updater because I was one of the many people who had problems with Apple's version 1.2 firmware . When I started using Rockbox I uninstalled iTunes and put iPod Updater 2006-06-28 on my machine.
I think I may have corrupted my iPod when I had my Sansa e280 connected at the same time as the iPod. I had problems in the past when I had my Dell Digital Jukebox connected at the same time as the iPod (pre-Rockbox days).