Is it formatted for win (FAt32) or for mac(HFS).
PC's can't read HFS but mac's have no prob. on Fat32.
But in a final note the iPod could be NTFS, in this case OS X can read but not write to the iPod. My advice would be to use Fat32 if you are using win and mac but you need to use a PC to upadte if usinf Fat32