nick.hortovanyi,
I don’t know why you’re having this problem. It is odd. Could you try flashing from a different host PC, using a different USB port on the PC, and using a different USB cable for the Jetson flashing port? Do you have a USB hub between your PC and Jetson; if so, perhaps try connecting Jetson directly to your PC.
Finally, perhaps if you run the following command it will give us more clues what is failing, although the failure is so early on this may not help:
sudo bash -x ./flash.sh jetson-tx2 mmcblk0p1
… and then send us the output from that command.