I think it is definitely a RAM problem. 2 million triangles is going to need about a gig of ram, and with undo, inevitable memory leakage, etc, it is going to already be getting very close to the 2GB memory limit (which is closer to 1.7 with all the memory needed for DLLs, etc)
I think this is why you are seeing the crashes while saving. The OBJ output allocates some large internal buffers to deal with writing out quad meshes (which aren't used for writing out tri meshes, but they are still being created!). So, it is highly likely that it just ran out of memory. Did you see the "sorry meshmixer has crashed dialog?" when this happened? If not, that is the most likely cause...this could also have been the source of your brushing crashes.
You might want to try reducing triangles once in a while. There is a tutorial video here: http://www.youtube.com/watch?v=1w9wPa4xGa4
I'm going to post a link for you guys tonight to a testing build w/ autosave. So, that should make your life better =). Also this version will have open/save for a binary file format, so you'll be able to save frequently without having to generate an OBJ each time.