Hmmm,,,, So Macbook Air users are skipping the extra? Can't imagine wanting to take up that much room on an SSD. I'm not concerned about other apps accessing the files. They won't et to access them if I don't download it. Is it possible to download to hard drive, manually move to external, and then have Logic Pro X look at the external for those files?
2. Copy original 'Logic' Folder located in /Volumes/"DRIVE NAME"/Application Support/ on the desired external drive and location.This is the location where all your additional content is beeing installed to.
3. You can delete the original folder or rename it and use it as a backup.
4. Use the symbolic link tool to create a symbolic link from the 'Logic' file which has been copied to your external drive.
Place the link (which looks like an alias) to its previous destination (/Volumes/Macintosh HD/Application Support/) and rename it to 'Logic'.
5. After this you will be able to download additional content.
6. Be aware if you want to download 30 GB of content and your SSD has just 20GB free space, it will show you the same error message because it stores the download file still on your SSD until installation is done. The solution for this is to download everything part by part.
Maybe there is a simpler solution. This was very quick and easy for me. Logic 9 and 10 both work still.