Long story short: Just got my M8 so it is 100% stock, no mods, nothing. I copied my pictures and music from my M7 to an SD card and popped it in the M8. Go to load up the HTC music app, nothing. Go to Google Play music, still nothing. Go to Gallery, even more nothing.
The weird part is that ES File Explorer recognizes and can read the SD card just fine (Not write but I know that's a whole separate ball game), the storage section in settings reports everything correctly. I can even take a picture and move that to the external SD card using the Gallery app and it'll come up just fine, but I then tried moving one of my old pictures I had taken on my M7 into the same folder as the lone picture I took with the M8 and nada, only the new picture shows up. Any particular step I'm missing here?