Apple iPod nano (5th generation) - Manual (Page 14)

14 Chapter 1 iPod nano Basics
If you see one of these messages, you must eject iPod nano before disconnecting it:
If you see the main menu or a large battery icon, you can disconnect iPod nano.
If you set iPod nano to manage songs manually (see “Managing iPod nano Manually”
on page 30) or enable iPod nano for disk use (see “Using iPod nano as an External Disk”
on page 84), you must always eject iPod nano before disconnecting it.
If you accidentally disconnect iPod nano without ejecting it, reconnect iPod nano to
your computer and sync again.