On OS X 10.9, GarageBand got upset when it couldn't find the Artist Lessons and Basic Lessons directories (folders) under Learn\ to\ Play when the external drive was disconnected.
To solve the problem, I copied Basic Lessons to the external hard drive (in my case, the external drive was called "My Passport Edge for Mac", and the full path was "/Volumes/My\ Passport\ Edge\ for\ Mac/Library/Application\ Support/GarageBand/Learn\ to\ Play/Basic\ Lessons"). Then I linked to each lesson individually. In the terminal window, I changed directories to the basic lessons folder:
cd /Library/Application\ Support/GarageBand/Learn\ to\ Play/Basic\ Lessons
and ran this shell script command to do the linking:
for name in /Volumes/My\ Passport\ Edge\ for\ Mac/Library/Application\ Support/GarageBand/Learn\ to\ Play/Basic\ Lessons/*; do base=`basename "$name"` echo $base sudo ln -s "$name" "$base" done
(You'll need to substitute the correct path to your own copy of the Basic Lessons folder, of course.)