Hello,
i want to have more than one iTunes library folder. I have the standard one on my macbook and no I want to have another one on my external disk.
Go iTunes -> Preferences -> Advanced (General)
Where you see "Itunes Music folder location" click the [Change] button and select your music folder on the external drive.
This is not possible with the iApps (iTunes and iPhoto). This is a limitation of the current approach taken by Apple for their file management in the iApps.