Help with files just disappearing.

[les7699]

I have an HTC One M7 and have posted in that forum with no answers so hoping someone here might know. I have uploaded all my music to dropbox (thank God). But my music keeps disappearing off of my phone. I didnt realize until last night to what extent. When I went to bed I had over 12 gb of music. When I woke up this morning I only have 8. What could be causing this and how can I stop it? HTC support said they never heard of this and didnt know what to do except a factory reset and that would mean having to re download all 2800 songs from dropbox. Who has time for that? And who knows if that will even correct the problem.

And yes I have ES file manager and the songs are not in any file anywhere on my phone.