Trouble Accessing Music Files After Recovery Operation

Try this approach:

1. Disconnect your external drive.
2. Select the tracks of an album you are aware isn't working properly, right-click, and use Remove Download.
3. Close Music, reconnect your external drive, launch Music.
4. Use File > Add to Library to import the same album from your external drive.
5. Confirm that you end up with one copy of each track that plays correctly, and that things like play count, date added, etc. are still correct.

If the approach above works you can repeat with the whole library. Note however that any content that comes from the Apple Music subscription service cannot be reimported from local files, so if any of those are broken you will need to clean those out manually from the Apple Music folder and then redownload.

tt2

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, 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

Use the Songs view. Exclamation marks may not show until after Music has determined that a file cannot be located. If you're certain files are where they should be see Repair security permissions for iTunes/Music for Mac - Apple Community and check that the permissions are set correctly.

tt2

Presumably you have a subscription to Apple Music. Drag the original copy of the file into the library. It should merge with the cloud copy leaving one entry in the library which is connected to the local file rather than the cloud. Assuming that works you can drag in the rest of your local content to fix it. Start with one or two albums as proof of concept.

tt2

After a track starts to play check the properties again. Is it connected to the same file location, or does it now say iCloud?

tt2