To be honest, I didn’t really try to determine what exactly was the issue.
I figured it failed on my desktop the first and second time because it got lost in the forest of USB devices I have plugged in. That PC did NOT recognize the CHIP plugged into any USB port even though he flasher reported a good FEL.
Second time on another PC (Surface 3), it was picked up immediately as a USB device, but there was no real indication of why it may have failed to see the CHIP after the flash. The subsequent try worked. And it worked a fourth time when I re-flashed.
This may be nothing, but try this… Hold the power button in while you apply power with a power supply, with the FEL jumper in. It should immediately light up. Let it stay on a few seconds then hold the power button in til it shuts down. Then plug in to your PC, let the PC find it, and run the VBox again. That’s really the only thing I did out of sequence and it worked after that. Not scientific, and probably just a fluke.
The cable I used successfully is a Kindle white data cable.