The other day I noticed that the contents of all my playlists were empty! The music (which is on my SD card) was all there, and the names of the playlists were also there, but none of them had any content. I spent several hours manually redoing the playlists (fortunately most of them were also on my computer so I had a guide to go by). Then I did some research on the Internet as to how to avoid this problem in the future, and I downloaded an app called Playlist Backup. Lo and behold, this morning my playlists were empty again. No problem. I opened Playlist Backup and restored them all in a matter of seconds. This could happen to you, and it can be really frustrating if it does, so my advice is to download that app (there are similar ones if you don't happen to like that particular one) and back up your playlists using it. Boy, did that save me a lot more frustration and headaches this second time my playlists turned up empty! My phone is an HTC One M9, 32 GB internal storage, 128 GB storage on SD card.