Synced with another iTunes library

[piatti]

I tried to put three songs into an iPhone but I have to 'erase this iPhone' and sync with this iTunes library. I would have to backup to preserve all the apps and other info right? Or if I back up with the original again, would the songs be gone again?

Also how is 'manually manage music and videos' different from 'sync only checked songs and videos'?

 

[Volkan1984]

You can only sync an iPhone with one iTunes library. Do you regularly want to put music from different iTunes libraries/ computers on your iPhone?

As for the second question, I can't tell you the answer...