Dear Mailinglist ...
The mirroring ist done but as soon as i try to reboot my system, it
doesn't starts up and im entering the "grub recover"-mode.
Where you can't boot before you do the mirror, it isn't tied to the mirror. Can you let us know what error you get when you boot.? Sounds like you might need to run update-grub so that it scans and makes a new /boot/grub/grub.cfg. Also do like Bob suggested and make sure your initrd is updated. Make sure /etc/fstab is correct as well. I would do these before update-grub as I believe update-grub relies on both to be correct when it scans.