I have a new linux mint install and have downloaded gmb (great browser). After gmb install I need to populate my music library. Following gmb instructions, I click on settings, library tab, + add folder, but cannot find my external drive (where my music is stored). I have exhausted all efforts (of my limited IT capacity), but cannot locate my external drive. Please advice

How did you install it ? It seems there is a flatpack version that creates problem with accessing files outside the home, if that is your problem, maybe try creating symlinks from your music to a folder in your $HOME, or try installing it without flatpack.
If you aren't using the flatpack version, other than a permission issue I don't know, maybe some screenshots could help.