GarageBand Library on External Hard Drive

I've moved my GarageBand sound library to an external hard drive and created a symlink from there in the Library/Aplication Support. This seemed to work at first as I was able to select the instruments I require. However, every time I open GarageBand it insists that I need essential sounds and starts downloading them. If I let the download complete it writes over the symbolic link and creates a new folder called GarageBand in Library/Application Support thus cutting me off from the external disk. Does anyone know of a way of avoiding this?