I have iTunes music files on external drive. How do I integrate into Apple Music?
How do I integrate music files on an external hard drive into Apple Music's cloud?
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See Make a split library portable - Apple Community for some background on the structure of an iTunes library. Anything you play with iTunes from your computer will be added to the library. Depending on your choice of media folder and the advanced preference for copying files into the media folder iTunes will either be connected to the original file or it will make a copy inside the media folder and attach to that. Typically you would add (import) media to the library using File > Add to Library rather than randomly playing files to add them. Once content is in your library, and you have Apple Music, those tracks should be matched or uploaded to your iCloud Music Library, making them available to stream or download to any other computer or device that is signed into your account.
You might want to make two libraries, a portable one on the external drive that is connected to your files, and a local cloud only one on the internal drive so you can be independent of the external drive and still have access to all of your music when you're not at home.
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