I did this using the last OpenPli image.

1. Format usb drive on pc.
2. Connect it to the Dreambox. If it does not recognize power off and then on again.
3. Go to Menu-> setup-> system-> harddisk-> initialization-> red button (wait may take a while).
4. In my case I restarted the DM.
5.After initiating press the blue button (PPanel).
6.Select Extend DM rootFS and ok.
7. Confirm everything and wait for 15 to 20 minutes.
8.ATENTION after completing the last step (7) restart the DM.
9. To verify that everything is correct via telnet use the command "df -h".
10.Be happy.