iPod doesn't appear in iTunes after being used as Windows hard disk
Open Disk Utility (/Applications/Utilities/Disk Utility), click on your iPod in the sidebar. Select "Erase" from the tabs in the center. Enter a name for the disk, then from the dropdown, select any of the 'Mac OS X' options.
Click the "Erase" button in the right hand bottom corner. This will delete all your data on the iPod
Wait for it to finish formatting, and you should be good to go!