Reports online indicate that Google’s Play Music app isn’t playing nice with downloaded/stored music. Specifically, music stored or cached on a Micro SD card is vanishing or being wiped from memory every time a phone is rebooted, or the memory card is removed and reinserted …
Google confirmed the issue in a support response to a customer who had experienced the issue. After initially dismissing it as a copyright protection feature, Google said that the issue had been identified and was being resolved (via Android Police).
Music files that are downloaded to a device (phone or tablet) are only downloading a cached version of song’s file, not the actual .mp3 file. This is done to help minimize the amount of space that the downloaded music takes up. This is also implemented to curb abuse of the subscription service as the download music is only accessible through the Google Play Music app for that device and account. This means that a user wouldn’t be able to download music to an SD card and then take that SD card and use it on a different device as the music is chased to that device and account.
With that said, music that has been downloaded for offline listening, either to the internal storage or SD card, should never be deleted without the user initiating the deletion. This is an actual known issue with this service. The good news is that we are aware of this issue and are actively working to get it fixed.
What’s interesting is that it only seems to affect the All Access Music. Since this is only supposed to play using Google’s own music app, there are some protections in place to stop it from being played elsewhere. It seems it’s these protections that are malfunctioning, stopping the music from being detected completely. Music stored in your own library on the memory card is a different story. Since you can technically play it on any device, this isn’t being affected by the bug at all.
If you have experience of the bug, be encouraged that Google knows what the problem is and is working on it. There’s no specific timeframe mentioned for when a fix will be implemented, but let’s hope it’s not too long.