I'm not exactly booting into fastboot -- that's the only thing that the phone will go to. It doesn't go to it from a menu. I'm not sure how it gets there. When I plug the phone into the factory cable it shows a green light and the computer gives a "connected" sound. The green light goes out and I usually press and hold the sound up and down with the power button until the green light goes on again. The fastboot screen eventually comes up even if I don't do that. I tried flashing with the instructions for your link. After removing the "get var", "var" and "oem" lines from the rom, rsd lite 6.1.5 and 6.1.5 no longer gave the "unknown fastboot command" error but went back to the "failed to flash gpt.bin". I'm wondering if there's some way to erase everything and completely reformat the whole phone. There has to be something on it that's preventing it from being flashed.