I think the firmware update went fine, probably the transfer of the two files you copied later caused the problem.
Is it stuck with the usb image even if is it not connected to your pc?
You can try to use another cable, it made the trick with mine 1200 .
Wait for the battery to run down and then try to reconnect it usb-
If you can regain the usb mode , delete the 2 files:
If you can get the preboot mode ,make a service cure
[Only registered and activated users can see links. ]
for .JCV and .IMG
Let us know how it goes.
Garmin wont look into your nuvi, they will send to reflash and send you a refurbished one ,I guess.