I have an iPhone that was synced to an iTunes library. Then I upgraded my Mac, and forgot to backup the iTunes library and transfer it to the new Mac. Ever since, I have been manually managing my music by dragging and dropping it from the filesystem onto iTunes, and not "syncing".
And things have been working fine for the past few years. But now, after yet another Mac upgrade, and upgrading to iTunes 11, I can no longer manually manage my music on my iPhone. When I try to turn on "manually manage music and videos", iTunes says:
The iPhone “kPhone” is synced with another iTunes library on “kart's macbook”. Do you want to erase this iPhone and sync with this iTunes library?
I don't want to erase the whole phone -- apps, app data, app settings, synced data in apps, videos, books, etc. I just want to copy music onto the iPhone. I have already deleted the music that's already on the iPhone. How do I copy music?