Open a terminal window, and type "mount". Look for a line that contains the mountpoint of your Ipod (/media/ipod from the System Trace). Check if that line has "type vfat" in it. If not, you formatted the Ipod with the wrong filesystem, and Rockbox cannot work on it unless you change that. Rockbox requires a FAT formatted Ipod (shown as "vfat" in the "mount" output).