My iPhone 4 did not copy my music from my 3G.

I just activated it and restored from a backup. It seems to have synched my apps, podcasts, settings, etc. just fine but my music didn't transfer.

I suspect the problem is that most of the music I had on my 3G was no longer on my PC harddrive and thus no longer in my iTunes library. For reasons of limited space on my main hard drive, I store all of my music on an external drive that is often not attached to the pc.

Is there any way to get the music library from my 3G to transfer? Or do I need to manually go through my huge library on my external drive, add it to iTunes and then add that to my new phone? That would REALLY suck if so...