Gen2 uses different system software (which is needed for the modular installable packages), has more RAM, and a slightly faster processor.
However, it also has no persistent root file system, which makes it painful to work around for some of the more “Fun” things you can do with it.
However, since it is a Gen2, and you want to hack it to run Plex, check out this thread.
Be aware that the single bay units lack sufficient processor power to do on-the-fly transcoding. Instead, you will need to tell plex to do the “alternate versions” thing, and let it grind away at the media library, and make multiple versions of your movies at different bitrates so that it can satisfy connecting devices and their needs.
In terms of limits with USB drive attachment, the manual mentions something about 3 drives on the port. This is most likely an issue with the automounter more than anything else. The real limiting factor is that the port does not give true USB3.0 speeds. It is more of a USB2.0+ port, where you get somewhat faster than normal 2.0, but not the speeds of 3.0. Also, CPU use will go up when using the port.
