I have a similar problem.
I tried to update nuvi 1200T with the newest CNE maps, but Garmin MapUpdater started software update first. After this update my nuvi shows only a Garmin logo.
I cannot access it as external disk, but it is recognised by WebUpdater and myDashboard (but only when I connect in this mode where you have to press upper left corner and power on). WebUpdater can see the device, but fails when it comes to updating (either 'connection problem' or 'update file corrupted' message).
I tried updater.exe and it seems to work - it shows progress bar etc. - but when the device reboots nothing is changed. All I can see is the logo.
Any ideas what's wrong and how to fix it?