- Completely Quit your Apple Music on your Mac.
- Hold option while clicking the Apple Music icon in your dock.
- Select Create Library....
Note that you can also Choose Library... if you want to switch between your created Apple Music libraries once you've set them up.
- Choose a name for your new Apple Music library and write it in.
- Choose where you want the library to be stored. (For example, if you want to create a library that's stored on your external hard drive, then make sure to have the external hard drive connected—and choose it.)
- Click Save.
Apple Music will automatically open up with your newly created library. Now, you just need to add the songs/media that you want to store specifically in your new library here.
Remember that to open back up your original library, you'll need to quit Apple Music and repeat step 1 to choose a library.