Just in case it helps. I had trouble updated my hardware 2.1 OS today as well. . .but on my Macbook Air. I found that I couldn’t have anything else plugged into my usb ports, or it wouldn’t be able to find the device. I suspect it has to do with the amount of power provided to each port. . .after unplugging all USB devices, I re-entered the special B2 mode and didn’t detect (this bumps my OS out of B2 mode), but manually picked my device level and firmware level. . .then it worked.
Maybe that will be helpful.