I would be interested to know if anyone has got this to work ?
I have been struggling with it today on a PiZero with a clean Rasbian image updated this morning. With the first attempt apparently there was no enough room on my SD card [ is was just the same size 8G as the main one]. Then the second time it just seemed to hang and when I clicked on cancel it partially locked up the Pi refusing to let me open a new terminal window to cleanly shut the Pi down - even if I went in through SSH.
It would be great to hear some positive experiences then maybe I can figure out what I am doing wrong....
*** The issue seems to be that I was trying to make a copy from an updated rather than fresh image ***