you should try:
1)Kunix's cure:
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2)format
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3)reinstall firmware
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4) run webupdater
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infos about folders and files
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You just need Garmin Device.xml to populate your Nuvi, Webupdater and some downloads from forum will do the rest.

If Kunix's cure doesnt work, I guess you must send it to Garmin
good luck

Edit: read also
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