Apple Music (mac OS) - ID3 metadata not transferred to new system

The default layout for the library has the database stored at ~/Music/Music/Music Library.musiclibrary with the media stored in ~/Music/Music/Media, where ~ is your user's home folder. When the library is in this layout you can move or copy the bold music folder to any new path or computer, then press and hold down option when launching music and use the choose library button to access the .musiclibrary file, and the library should work. You're opening a specific library, rather than importing media to a new one, so all metadata should remain in place, including details such as ratings, play counts, playlist memberships, etc. that are not captured in tags. If the library is currently in a split layout, where the database is on the system drive and the media is on an external drive, then it should be possible to rearrange the library into the nested relationship of a Music folder containing the .musiclibrary file and the Media folder in advance of moving the library from one computer to another. On the new computer there is no need to separately import media files and playlists, everything comes over at once, and because the database is on the external drive you get a warning if it cannot be opened when you launch Music, which avoids the problem of the media folder getting reset when you have a split layout. Note that Music > Preferences > Files tells Music where you're going to store new media in future, not where existing files are, nor which database file to open.

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See Repair security permissions for iTunes for Mac - Apple Community. An obscure but potential explanation might be multiple tags in mp3 files. See also Move your iTunes library to a new computer - Apple Community. When moving or restoring the entire library all the metadata, playlists, ratings, playcounts, etc. should be preserved, so you don't have to rely on embedded metadata, even though you still want this in case you want to move things between libraries.

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What is the media folder location as defined under Music > Preferences > Files? For Music an ideal location would be <External>/Music/Media with the Music Library.musiclibrary database stored in <External>/Music. This layout would make it easy to move the library from one computer to another, backup as a unit, etc. Storing the database on the external drive avoids an issue with split libraries where the media folder can silently reset to. the internal drive if Music is launched when the external drive isn't ready.

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If your media folder is on the external drive then using the Copy to... function won't impact the internal drive.

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