How to move iTunes’ Mobile Applications folder to an external hard drive
This may not be the answer you are looking for but if you simple want to move you entire itunes library contents onto an external harddrive the simplest, best way in my opinion, is to move your entire ~/Music/iTunes folder onto the external drive then point iTunes to open the library in that location. Its as simple as closing iTunes, moving the folder, and then while opening iTunes again hold the option key (shift key for windows). It will then prompt you to choose which library you want iTunes to open. What you're really selecting is the iTunes library.xml file, but you can select your itunes folder and it will find the file. 

If you want to experiment you can do the same thing while opening itunes and create a new library in a new location to play with.