I went to exercise the other day and my iPod Nano said "Disk Mode" and the Bottom said "OK to disconnect".

Wasn’t sure if it was “dead” or what. What worked for me was to hold down the Menu and the Click Wheel *the middle round wheel part* for about 5 or 10 seconds and voila it was fine.

This article explains how to put it into this mode manually http://support.apple.com/kb/HT1363 must have been while I was traveling it was in my bag … glad it wasn’t broke!