iTunes Media, iTunes Music, and Music/Music Library HD change

With Music closed copy MacHD/Users/me/Music/Music to the root of the external drive.

Press and hold down option as you start Music, then use the choose library option to connect to the new copy.

Check Music can play your files, if not you may need to change the media folder, or swap back to the original library.

Use Music > Preferences > Files tab and reset the media folder to <External>/Music/Media.

Consolidate if prompted, otherwise use File > Library > Organize Library > Consolidate Files.

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Apologies, I left out some housekeeping steps. The consolidate operation copies everything from the internal drive out to the external. Once you're certain everything is working correctly from the external drive then you can delete the original MacHD/Users/me/Music/Music and MacHD/Users/me/Music/iTunes folders, but I suggest you look at Managing your Mac media libraries - Apple Community first. You may want to migrate any video content in the TV app to the external drive in a parallel fashion to the Music library, and check there is no other old media that you want to archive before you remove the old iTunes folder.

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