Not sure if there is a way to work around this, but it would be best to go ahead and reformat it for mac, especially if you have already moved to a mac.
Of course, you will need to make sure you have moved your music, playlists, etc... over to your mac (i think there are mac support articles on how to do this). After you format it for the mac, it will sync with your Mac's iTunes and be back up and running smoothly.