In 2.78 I've tried exporting one of our models - it's triangulated and has a root bone. The model exports as a 1kb file and is not visible in the viewer. I'll have to download 2.77 and try that.
Using my md5 exporter?
Did you press Init button in your Properties to have add-on initialized and mark deforming bones as exportable ?
I believe I have a video tutorial about how to use md5 exporter.