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.

iTunes lost the ability to follow symlinks when organising media a few updates ago (see this answer of mine on Super User). As you noticed, it will find its assets in symlinked folders, but it will not put new assets there.

Considering you have already moved the brunt of you iTunes media, a simple solution may be to move the whole media library to the external drive, which is supported by iTunes: you’ll find the option in Preferences » Advanced (move back your symlinked folders to their source location before you do). See Apple’s instructions on this for a detailed how to.

If you absolutely must move only parts of your assets, there is another, somewhat less convenient way: iTunes will happily manage files stored anywhere by expressly storing the path to them in its library (which is exactly what it does when you uncheck the “Keep iTunes media folder organized” option in preferences). If you would like iTunes to manage your library but for some relocated part, TuneSpan will leverage this ability to link a subset of your files stored somewhere else into the library.

I manually moved the mobile applications directory to an external drive, then added the new location to my library. It asked to replace the existing entry in the library one at a time.

While the import worked, there are no entries in the library.xml file for some reason.