I have faced similar dilema with my 770, but it's very unlikely that is the 770 problem, do de device works alone, or it is in need of flash? If works try PC connection and transfer some files between the memory card and device memory. Also provide us with more info, what wersion of flasher, what windows?

But I missed something

In my case turns out that the problem was caused by badly working USB Ports. I turned few off through the BIOS and leaved only 2 (For mouse and USB Cable ) and now I can flash perfectly.