DON'T USE WEBUPDATE to update the device is all i can say...
I did this as I read some other comments that the Garmin webupdater application has (in some cases) caused corruption.
When I tried to do it again, I got the message that the 670 was already updated,tried to update again,got the message"The update file is corrupted".
But your procedure worked,took a good minute for the flicker and then the Pre-Boot message to disappear,then it started loading some Bluetooth update and then came back to the normal screen.
I hope this assists other if they encounter this problem. Strangely when it came back up the v2.7 upgrade had worked for me too.
I still don't know why the firmware upgrade process failed BUT after performing the above sequence I found that the firmware upgrade had actually worked (strange). Had the "system software missing" problem after attempting to upgrade software to v2.7 using the web updater. JW hi nick, thanks from me as well for your accurate and useful post - was having kittens thinking i'd d**yed up my brand new garmin nuvi 310, but your instructions have saved the day.