Not sure what's happening there, for now you can try creating a symbolic link on the main SSD that points to the G-Tech drive Applications/Band-in-a-Box/RealTracks and set the RealTracks folder to that symlink.
https://www.macupdate.com/app/mac/10433/symboliclinker
https://www.macupdate.com/app/mac/41493/symlinker

or just copy the contents of the Band-in-a-Box folder less Drums and RealTracks folders to SSD /Applications/Band-in-a-Box
run it, then see if you can point it to the RealTracks and Drums folder on the G-Tech drive.