Changes from Enable Disk Use to Manually Manage

I hope someone can help because I'm at my wits end here. My classic has started changing my settings from Enable Disk Use to Manually Manage music and when I switch it back I get a message saying this ipod is synced with another iTunes library on my Mac. I've tried erasing it and reloading and a couple times I was able to sync with no problem but then it screws up again. Can anyone tell me why it's doing this and how to fix it? I haven't been able to find any info on this problem on here so far.