telnet to the box and write the following command after loggin in:

cat /dev/mtd/3 > /var/tmp/backup.img

you can download this image to your pc, so you can always switch back to this backup image.

how to switch back:
first ftp the file to the dreambox in /var/tmp/
then telnet to the box and use following commands:

cd /var/tmp
eraseall /dev/mtd/3 && cp backup.img /dev/mtd/3