What is the path to your active Music Library.musiclibrary file? Typically this will be in ~/Music/Music where ~ is your user's home folder. What is the path to the media folder under Music > Preferences > Files? Presumably this will be something like <External>/iTunes Media or <External>/iTunes Media.
As long as the .musiclibrary file is on your internal drive you can use Music > Preferences > Files > Reset to update the media folder to Media in the location of the .musiclibrary file, and then File > Library > Organize Library > Consolidate Files to copy over the files from the external drive.
If the .musiclibrary file is on your external drive please back with its location and that of the media folder for further advice.
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