Apple - Support - Discussions - How to make a back up copy of system ...

There is a way but it's complicated.

1.) Get an 8GB USB stick and format it for a boot disk (partition it with GUID option and (Mac OS X extended (journaled)).
2.) Insert the Apple USB stick also. There is a invisible file at the root you need to mount on the desktop called MacOSX.dmg. You can use a utility such as Onyx or use the terminal with the open command to mount it.
3.) Use carbon copy cloner to clone the mounted "Mac OS X Installer Image" to the new USB stick
4.) Unmount the "Mac OS X Installer Image"
5.) Use carbon copy cloner to then clone the Apple USB stick to the new USB stick. Select the delete exiting files. Steps 2 - 4 are necessary because if you just clone the Apple USB stick the new USB stick won't be bootable.
6.) After this finishes you will now have a bootable copy. You can then also use Disk Utility to create a disk image of the new copy.

I've done the above and it does work.

如何備份 MacBook Air 所附的 USB 還原 stick。