Copy/pasted music to iTunes, now it says it's synced with another library...

The vast majority of my music is on an external hard drive and I was having no luck importing it into the new iTunes, so I just opened a Finder window and copy/pasted some music I wanted into the play list I use to sync music to my phone. I then went in to sync and got this message:

"The iPhone “xxxx iPhone” is synced with another iTunes library on “xxxx MacBook Pro”. Do you want to erase this iPhone and sync with this iTunes library? An iPhone can be synced with only one iTunes library at a time. Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library."

I haven't changed to a different version of iTunes...just added some songs. What the heck? Now what do I do?