From the descriptions here, I am having the same problem, namely Poweramp vs. the Marshmallow SD mount point. My music shows up in Poweramp, but I get a message saying "Folder is missing/not readable. Try full rescan?" Poweramp worked fine until the Marshmallow update on my LG V10 from Verizon in the US. Other music players work fine. The most recent update didn't fix the problem (2.0.10-build-588). I am not an Android developer. I know I've seen mount points, but I can't recall how to show the Linux path. If someone can explain a way to show this, I'll be glad to verify.