I have this issue with X-plore. It was working perfect, but I accidentally turned on "Use X-plore USB-Driver". It asked me to mount some of my hard drives. I chose yes, but then all of them disappeared from the list. So I just disabled the feature and restarted the nvidia shield. Now all the three drives show and are browsable on X-plore (2 of them connected through a usb hub), but no other app will play any media on them. The other apps can see and browse the files on the drives, but the apps have no access to the files. I don't know what to do to fix this? I plugged off the usb hub and put it back on. Restarted multiple times. I even uninstalled X-plore.
It got fixed. Not sure how exactly. I uninstalled and reinstalled X-plore a couple of times, restarted the shield a few times, and reinstalled the other apps. The apps affected were Nova Player and TV Photo. I'm quite sure X-plore caused something to go wrong with that option..