Comment 2 for bug 829407

I think this is exactly the problem.

One easy way to reproduce it is as follows:
1.) Insert an USB pendrive with btrfs on it.
2.) Verify that the USB pendrive got automounted (but do not open it in nautilus)
3.) dd the pendrive (depending on your device number, i.e.: dd if=/dev/zero of=/dev/sdc)
4.) remove the pendrive
5.) Try to shutdown Ubuntu