I have the WD My Passport Wireless Pro and loaded up my files from my Plex server. This device just choked on the files and could not play them. I went back and forth with WD level 2 and he suggested that it was because I had the 5.1 audio and no stereo in the streams. I thought is was the size of the video but he said that should not matter. I then re-encoded a bunch of my videos and added that stereo channels (and selected them during playback). I still can't get movies to play consistently.
I have tried my Kindle Fire, Nexus 5X, and iPhone 6s. All have the same problems. The Plex app has a hard time seeing the server. When it does it will not play the movie. I have much better luck using a web browser. But even then it will not play well. Most of the time, if I can get it to play the movie, it will get to about the half way part of the movie then just stop. No errors, just goes back to the Plex menu. Is there some magic video size and audio track that I need to encode this files? Note that if I turn off the Plex server and use the Twonky DLNA Media server I have MUCH better luck playing the files.
Also note that when I am trying to use this device I am in an airplane with no internet if that matters.