"The main USB port" in this context just means the USB port on the PC itself. The clue is in the "not a hub" part. One thing comes to mind. Until the last two posts I did not realize you were doing this in Windows. If you are using Windows 7 or newer and/or any 64-bit Windows, then you may need some special considerations. Look up one of about two dozen threads on the topic. None of the suggested solutions have ever worked for me though. The only thing that worked was booting a live, 32-bit Debian or Ubuntu CD and flashing from that.