The circumstances were the same all 4 times: I was copying large directories between different ext4 partitions (using nautilus) when the system locked up. The directories in question have tens of thousands of small sized files. I am going to mark bug 334581 as a duplicate of this one so that we can focus our discussion and testing on one bug report.
I had experienced the same, and reported it at https:/ /bugs.launchpad .net/ubuntu/ +bug/334581. I have had to hard reset my computer four times today.
The circumstances were the same all 4 times: I was copying large directories between different ext4 partitions (using nautilus) when the system locked up. The directories in question have tens of thousands of small sized files. I am going to mark bug 334581 as a duplicate of this one so that we can focus our discussion and testing on one bug report.