Screenshot is from iTunes v4.6 but you will still get the idea.
If you have manually manage songs and playlists selected then enable disk use is checked and disabled because it is needed for dragging and dropping to the iPod.
If you are using manual update then that is the reason the iPod is not unmounting after you have updated music because you need to right click the iPod and select Eject. If you do that and the iPod stays at do not disconnect but is not detected anywhere on your computer simply reset it by holding down the Menu and Select buttons together.