Flash any image on Dreambox (DM520, DM900 and DM920 etc), I explain here for DM520

1- Format USB with Fat-32
2- Label usb "DREAMFLASH"
3- Copy image to USB Drive
4- Rename image "dreambox-image-dm520.tar.xz"
5- Insert USB to receiver
6- Now from standby menu select Recovery Mode
or Shut Down your receiver and then press and hold touch button of receiver
or from telnet give any one of below commands

to-the-rescue
or
echo "rescue" >> /proc/stb/fp/boot_mode
reboot

Note:
If receiver is on red light then first Power Off your receiver from rocker switch.
Then power on your receiver and touch and hold front touch panel within 500 ms to bring receiver into Rescue Mode