The Passport Wireless Pro is not capable of transcoding video. CPU's just way too weak for that. When I installed Plex, it disabled Twonky so perhaps you can only have one of them running at a time. Right now, I only have 10 videos in the Passport's Plex library. As I mentioned earlier, I'm still in the process of putting it through its paces not to mention I still need to convert my existing library to mobile friendly format for Direct Play. Dumping 1.85TB in one shot may be too much for Plex to process on the dual-core ARMv7 CPU.
You're correct that it will only run Twonky or PLEX at one time. I didn't turn on Twonky until PLEX just didn't function anymore. It's funny that it didn't have a problem with the service when I transferred 1,615 video files which amounted to 1.74TB, it was when I loaded my music which was 4,890 files and 80GB that Plex stopped working. Maybe I reached a limit, but I'll put a incident ticket in with WD and see if I can resolve the issue. If Plex can't handle the media library with the current hardware, I'd rather know what the limit is. Even if Plex can't handle my library I'm still loving the Wireless Pro, Twonky has no problem with the library size and with InFuse I'm able to stream AVC Bluray quality video without stutter on my iPad Air 1. Now I'm able to take my entire media library with me for travel without needing to bring my laptop and external drives. Awesome!