How do I move GarageBand's Lessons and other libraries to another drive?
Quit GarageBand
Move /Library/Application\ Support/GarageBand/Learn\ to\ Play/ to another drive
Open Terminal
Type ln -s followed by a space
Drag the folder created in the second step into the Terminal window to insert the path
add /Library/Application\ Support/GarageBand/Learn\ to\ Play and press Enter


Test it both the external drive connected and disconnected to be sure GarageBand doesn't have a problem with the LearnToPlay content not being available.