How can I restore the connection between Apple Music and my external disk on MacBook Pro?

I have a MacBook Pro (late 2012) that I use for music in iTunes. The music library is in an external disk. After updating to Mojave (10.14.6) there is no connection from Apple Music to this external disk and therefore I can't listen to any music. Is there any way I can get it back? Can I go back to my old macOS (it was some 10 point something, probably a Mountain Lion or somewhere in that vicinity)?

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