is the usb connection actually dropping? loose cable at either end? is the usb icon still up on the device?

Is there another usb device causing a hiccup? I know I have a usb mouse that polls my usb like crazy. I switched to a ps2 mouse for flashing. Try disconnecting all other usb devices.

Just keep trying. I had never flashed before this week and probably tried at least 30 times over the past 48 hours, but my device works now. I know how frustrating it is. I almost decided to just preorder a N9 at one point. I saw bare minimum installs (no OS) and reboot loops, successful flashes that actually were not... but I never encountered your error.

There has to be something to it. Don't give up.