When I plugged in my Nexus 7, a few weeks ago it didn't mount. So I went searching for a solution. I ran accross this post: How To Mount The Google Nexus 4 on Ubuntu
by Joey-Elijah Sneddon and it worked.
End of story, NO! Last night, the Nexus would no longer pop-up when plugged into a USB port. So I went searching again for the solution, but could not find it.
After a while, I figured out that the USB cable was bad. A cat had chewed on it.
Bought a new cable. Presto everything worked again. Not only did everything work again, but I once again found Sneddon's post.
In searching for a solution, I had found many posts asking for a solution along with numerous solutions; but the one posted by Sneddon appears to be the easiest to install. So I am leaving this post behind with the hope that it will help others.