same happen to me, what I did to return to the original iPod frimware was:
- using the DISK MANAGEMENT delete volume (with the iPod conected via USB cable), you look whats your iPod HD then with right click mouse look for DELETE VOLUME
- after you delete volume, format the iPod (in the same DISK MANAGEMENT in WINDOWS) with exFAT
- unplugged the iPod
- plug the iPod and open iTunes, resorte the iPod using iTunes
cheers