Doing it his way does not break on content downloads
For GarageBand just replace Logic with Garage\ Band
mkdir /Volumes/{External Drive's Name}/Library
cp -r /Library/Application\ Support/Logic /Volumes/{External Drive's Name}/Library/Logic
sudo rm -rf /Library/Application\ Support/Logic
sudo ln -s /Volumes/{External Drive's Name}/Library/Logic /Library/Application\ Support/Logic