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Lets try a master reset on the Nuvi first, since its been unused for some time. Please watch the 1 minute video below and complete the steps it shows.
Once thats complete, connect the device to your computer again. Then, if its been more then 3 moths since you downloaded a fresh copy of garmin express. Then update garmin express by downloading and installing it from this page.
Once installed, run garmin express. Make sure it recognizes your Nuvi 50 and then try the update again. It should work now.
But if you still run into issues, youre welcome to respond and I will assist you further as needed.
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