Locations of Library Files to Potentially Clear Out Go to “GO” in the top menu bar and hold down “option” so that “Library” is revealed. Go into Library... Application Support and look for MobileSync and SyncServices. You can safely clear the contents of both provided you have your devices (iPhone/iPad) backing up to iCloud.
Location of Apple Loops (Do not remove if you may ever use Garageband) Macintosh HD...Library...Audio...Apple Loops (also GarageBand)