I plugged in my Nuvi 2457 to update the maps last night - we're close to the US/Canada border and don't want to incur roaming charges every time we cross over for a few hours. My Garmin Express software was failing during the map update, it was literally turning OFF after 30 seconds into that phase of the map refresh process.
I had plugged my Nuvi into a USB hub - this was the issue, when I plugged it directly into a USB port my laptop the update completed without issue (after at least 5 failures).