Just copy all the files and folders from Nuvi to your pc. Be sure to enable Windows to see the system files as well as the hidden files
Read here in post n5
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If you can do this too.
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Like Neil wrote probably something went wrong with Express, Mapupdater or Webupdater, or you didn't unplug it safely from your pc and may be a file got corrupt, or simply, you copied something (a poi for example) not suitable in your Nuvi.