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Location: Perth, WA
That's expected behaviour. Your USB is mounted by some other process and it's mounted outside /media/music or /media/playlists.
The browse folder only work within /media/music and /media/playlists and not any other location. The browse section is really just do to copy files quickly.
To see the contents all you need to do is to mount the USB inside /media/music and it'll be visible from there.
To manage other locations there are other better file managers out there.
Snakeoil Operating System - Music, your way!