Following right on from this comment about good signposting, in May 2016, Vellum Atlanta lost a music library to Apple Music, where the material on their local drive was replaced by material from Apple Music, and then ceased functioning when the trial subscription to Apple Music stopped. Thankfully, they were able to restore a backup, but it was not a happy time.
Serenity Caldwell suggests that there was confusion about what actually happened, and provides a tweet from Jason Snell with a dialog box that may have been the culprit.
This dialog box is a nightmare waiting to happen. There are two options to get rid of something on one dialog box, one which will delete the local copy of a thing, one which will remove the reference to Apple Music. Guess which one gets rid of the local copy?