I had the same problem as the other people in this post when trying to dump/flash my BenQ. As people mention, it all has to do with pointing to the correct port.
I ran iPrep and it concluded I should use a certain port, no go. I ran dosflash (no params, which puts it into auto mode) and it tried using a different port than what iPrep chose. Still no go.
So, I needed to determine the correct port to use, so I fired up Sys Info. To do this, go into Windows, from a command prompt, run msinfo32 (System Information), then go to "Hardware Resources", then "I/O", then sort by "Device". Take a look at all the items that mention SATA. I personally have an Intel 82801GR/GH chipset in my motherboard and a VIA RAID Controller PCI card. The Resource number is the port (the left of the 2 numbers). I basically tried each till I hit the correct one (by modifying the generated scripts that iPrep creates and entering the correct port).
I found sometimes it would stay at 0xFF, 0x80, 0x00, etc. This just meant it was the wrong port. Sometimes it would act like it was communicating with something (by showing 0xD1 or 0x51), but I believe it was only communicating with my PC's DVD drive or perhaps a PC HD and not the 360's BenQ.
Good luck!