If you look at dmesg you might notice an error like
HFS+-fs warning: Filesystem was not cleanly unmounted, running
fsck.hfsplus is recommended. mounting read-only. This means that
the HFS filesystem on the iPod has been marked as inconsistent. To
mark the filesystem as consistent again, mount and then unmount it
using the HFS tools (hpmount /dev/sda; hpumount).