'Running ``CreateFilesystem`` on a filesystem mounted with
``MountBlockDevice`` fails in a non-destructive manner.'

def test_create_fails_on_mounted_filesystem(self):

    (scenario, mountpoint) = self._make_mounted_filesystem()
    afile = mountpoint.child('file')
    afile.setContent('data')
    self.failureResultOf(scenario.create(), FilesystemExists)
    umount(mountpoint)
    self._mount(scenario, mountpoint)
    self.assertEqual(afile.getContent(), 'data')
