I am the developer of an application called TuneSpan. It may be able to help you manage your library in a different way.
With TuneSpan you can keep your default media location the same, so new music get added to your internal drive, but you can move any media wherever you choose. Through TuneSpan you can select any media type, playlist, artist, album, tracks, etc, etc, and just select a location to move the files to. TuneSpan will copy the files, set the locations in iTunes, and clean up after itself, trashing original files and empty folders as well so your system stays clean.
TuneSpan is available on the Mac App Store, and you can check it out at http://tunespan.com
I hope TuneSpan can help you take better control of your iTunes media.
I may receive some form of compensation, financial or otherwise, from my recommendation or link. <Edited by Host>