I had problems with some 3rd party slideshow apps when I had the library on an external HD. However, I'm not sure I moved it there like I told you to do.
If you have Quicktime Pro there is a fairly easy way to add your music to the slideshow. It goes like this:
1 - export the slideshow from iPhoto without the music.
2 - if you have more than one music track to use join them together into a new file with an audio editor like Garageband,
Amadeus Pro or Audacity.
3 - open your music track with QT Pro 7.
4 - Open the slideshow movie with QT Pro.
4 - in the slideshow window type Command+A (select all) and then Command+C (copy).
5 - go to the slideshow window in QT Player and select Edit->Add to Selection and Scale.
6 - save the merged item as a self contained movie.
This will add the slideshow to the music and scale the slideshows length to match the music. Don't do it the other way around as the music timing will be altered changing the speed and pitch of the music.
With this method and if you have more than one track you can overlap and fade between tracks eliminating the dead air that often results between tracks. It also will assure that the music will end with the last slide. NOTE: I add a blank, black slide at the end of my slideshows so the last slide fades to black and the music ends on the black slide.