when you say that iTunes is missing songs, do you mean that only some of your tracks are missing or that all of your tracks are missing?
If the former, did you copy all the music from your old computer to the external or just the iTunes Library?
If all the music, reconnect the external and, in iTunes Preferences, consolidate the library.
I just got a new computer in may and I wanted to move my itunes music and stuff from the old one (That runs Windows XP) to the new one (That runs Windows Vista). But when I installed it and did all the settings it wouldn't find all of my songs (All of my stuff was moved to an external drive by dragging and dropping).Also, I moved all of the .itl and what ever files to the new computer. So now I have not been able to do anything with my ipod and itunes.