Music app does not sync with folder

1. Select a song, press cmd+i to get info, look at the File tab to see its location.
2. OK, SET A is the songs known to your library.
3. The media folder designated in preferences is where new downloads, rips, or conversions go. Music doesn't scan this folder to make the library match the contents of the folder.
4. When Music imported your old iTunes library it won't have removed the old database or other supporting files. So far, so normal.
5. So SET B is all the music content on your external drive, got it.
6. Music only knows about the content that has been added to it. Typically it will contain a subset of the audio content that was in your iTunes library, plus anything added since. Content that would likely have been excluded would be podcasts, audiobooks, tones, voice memos, and any audio in your media folder that wasn't connected to iTunes when you upgraded to Music.
7. The ideal location for a Music library on an external drive would be <External>/Music with the media folder being <External>/Music/Media.
8. See 3. above, changing the media folder tells Music where to put new downloads, rips, or conversions. Selecting an empty folder won't clear the contents of the library.
9. Revise your expectations, see 8.
10. Music uses a file typically called Music Library.musiclibrary for its database, not the iTunes Library.itl file or supporting files from iTunes, so deleting those can be expected to have no impact. The database is typically located in ~/Music/Music but if you want to host your library on an external drive you may want to move it to <External>/Music (see recommendation in 7.) then option-start-Music to select the new copy of the database.
11. Good. Managing content from multiple accounts complicates matters, particularly if you want to redownload past purchases. When you are signed into your Apple ID you may also see any unhidden purchases that aren't already included in the local library. These would have the location Cloud and can be download on demand. See the right-click context menu or the little cloud/download icon.
12. (Re)authorizing the computer allows your computer to play protected content. There shouldn't be any impact on the content of the library when you authorize or deauthorize.

You can use File > Add to Library to scan your drive, or any particular folder within it for new content to add to the library.

If you want to start a fresh library press and hold down option as you launch Music and keep holding until you are asked to choose or create a new library.

In a standard Music library the typical path of a track is:

<Some Path, e.g. ~/Music>/Music/Media/Music/<Artist>/<Album>/[D-]## <Name>.<Ext>

There is a trick to eliminate the Music folder inside the Media folder if desired, which requires editing a hidden .plist in the Media folder and changing a 1 to a 0, then turning Keep organized off and on again.

Music has a feature to copy tracks that are outside of the current media folder into it. File > Library > Organize Library > Consolidate Files. The drawback with using this feature is that files are copied rather than moved, leaving originals for you to clean up by hand. At the very least you probably want to add all existing media on the drive to the library first before you consolidate to any new path so you can be clear that anything left outside the new media folder after consolidation can be safely discarded. Of course you may also have some legacy video content on the device, which belongs in the TV app, which behaves much the same as Music, so you might not want to discard the entire iTunes folder until you secured that in a new media folder too.

See also Empty/corrupt iTunes/Music library after upgrade/crash - Apple Community which explains why you might see only your unhidden purchase history as opposed to your local content that had previously been part of your library.

Importing local content when you have copies in the cloud won't create duplicates. The usual reason for that is importing files located outside of the meda when copy files to the media folder is enabled. Neither iTunes nor Music keep a record of the import locations, so if you import from the same place again you get new copies. If you attempt to add a file that is already connected to the library then nothing happens.

tt2

If desktop, depending on the number of songs or damage to the database you may need to remove the database and let it re-interrogate your collection, see the command below, enter your user name in the xx section.

rd /S /Q C:\Users\xx\AppData\Local\Packages\Microsoft.ZuneMusic_8wekyb3d8bbwe\LocalState\Database

Also, confirm in groove settings that you are monitoring the music folder you're dumping songs into. 'Settings / choose where we look for music'

[Edited by Moderator]

You're addressing the wrong app on the wrong platform. The OP is using Music on a Mac.

tt2