If your drive is installed internally on IDE (pref sec master) make surre you have no disc's in the drive. Using Windows as OS, Go to Unofficial BenQ firmware page (top of forum), download WinDWflash and the latest firmware release in a ".cvt" file form (B7P9).
Make sure you disable any anti-virus or other programs that might start up while you are flashing.
Now run the flasher, loading the .cvt firmware file & then flash firmware in one step. You can run WinDWflash, from the safe mode if this feels better. Reboot and you have a DW1620 Pro!
Externally, I would recommend using USB 2 to flash, same steps, different firmware, 47L9 .cvt