Moving Music from Macintosh HD to external SSD, while preserving all playlists and metadata.

The method that generally works when the library is in the standard layout is to copy the whole library from the original location to some new path, then press and hold down option to choose the library file in the new location, and it should update its references to use the media folder within that library folder. Moving the media folder independently and then updating the preference setting doesn't usually go so well.

The "missing file" issue with exclamation marks happens if the file is no longer where iTunes or Music expects to find it. Possible causes are that you or some third party tool has moved, renamed or deleted the file, one of its parent folders, the drive it lives on has had a name change, or you've moved a non-portable library to a different path (see Make a split library portable for details). It is also possible that iTunes or Music have changed from expecting the files to be in the pre-iTunes 9 layout to post-iTunes 9 layout, or vice-versa, and so is looking in slightly the wrong place, (see the iTunes Media Organization section of Managing your Mac media libraries - Apple Community for details) or that you've been too aggressive when deleting duplicates at some point.

Select a track with an exclamation mark, use Cmd-I to get Song Info, then click No when asked to try to locate the track. Look on the file tab for the location the library thinks the file should be. Now take a look around your hard drives. Hopefully you can locate the track in question. If a section of your library has simply been moved, a folder renamed, or a drive label has changed, it should be possible to reverse the actions. If the difference between the two paths is an additional Music folder in one path then this is a layout issue. I can explain further if that is the case. If everything is where it is supposed to be try Repair security permissions for iTunes for Mac - Apple Community.

In some cases the library may be able to repair itself if you go through the same steps with Get Info, or when playing a track, but this time click Locate and browse to the lost track. It may then offer to attempt to automatically fix other broken links. Although it says something like "use the same location" I think it expects to find the tracks in the same artist & album layout they were in previously, with one systematic change to the path.

If you want me to try to provide specific advice please post back the following details:

1. The location of the media folder under iTunes|Music > Preferences > Advanced
2. The location of a sample missing track shown under Song Info > File > Location that begins file://
3. The true path to the file whose details you gave in 2

See also FixLinks - an AppleScript to repair broken links in Music - Apple Community.

tt2

The issue here is that you haven't started out with the library in the standard layout where the library database is ~/Music/Music/Music Library.musiclibrary and the media folder is ~/Music/Music/Media. One approach would be to revert to your original library on the internal drive then use my advice in Managing your Mac media libraries - Apple Community to massage the library into the usual layout, finally copying ~/Music/Music across to your external drive and accessing the library there.

That said I think it will be easier to work with the active library that is already on your external drive (which is what is going to reopen if you haven't successfully chosen or created a different library) then use the menu options Music > Preferences > Files > Reset Music Media folder location, then consolidate the library either when prompted after changing the media folder or via File > Library > Organize Library > Consolidate Files. This will create new copies of each file in <External>/Music/Meda and update the library to point to the new copy. Once complete the files in both the internal and external New Sound Files folders will be redundant.

N.b. If there isn't enough free space on the external drive, at least temporarily, for two copies of all of your music files then delete the external version of New Sound Files and empty the trash before consolidating the library.

tt2

Unfortunately, I have done this many times... and it's always NOT without issues. Also, what Apple told you about your purchased songs need to be located on an internal Macintosh HD is just not true.

Don't forget... in your Music settings under "File" to change your Music Media folder location to the new external SSD, with both options underneath checked. This should allow your Mac to find your library and organize the files.

However, for me, the best way to create more hard drive space for my music library without problems was to increase the "internal" storage and make direct copy of my Macintosh HD and replace it with the larger HD. Good luck!