Comment 12 for bug 726790

I also had the problem building a lucid image but the added sync line did not help.

I added the -l option (lazy unmount) to both umount commands in plugins/ubuntu/dapper.py and util.py and it worked and the devices got unmounted.

util.py (starting in line 194):
def clean_up_tmpfs(mount_point):
    """Unmounts a tmpfs storage under `mount_point`."""
    umount_cmd = ["umount", "-l", "-t", "tmpfs", mount_point ]
    logging.info('Unmounting tmpfs from %s' % mount_point)
    logging.debug('Executing: %s' % umount_cmd)
    run_cmd(*umount_cmd)

plugins/ubuntu/dapper.py (line 116)
(I left the sync in there, guess it'd also work without the line)
    def unmount_dev(self):
        self.context.cancel_cleanup(self.unmount_dev)
 run_cmd('sync') # added by me
        run_cmd('umount', '-l', '%s/dev' % self.context.chroot_dir)