Add feature to allow for external drives as library folders

[benedictdaske]

Is your feature request related to a problem? Please describe.
When I select a USB stick as the folder for my library, the path is always set to "/".

Describe the solution you'd like
I'd like to set external drives as a library folder the same way I can set an internal storage folder as one.

Describe alternatives you've considered
Maybe a soft-/hardlink from an internal storage folder pointing to the external media storage.

Additional context
None so far...