Thread: How does a sd card work in Garmin Nuvi255? (Regarding updating map)

Hello, I need your help regarding updating a map.
The device is Garmin Nuvi 255.
To update it to a newer version, recently I downloaded a North America map 2016(file name:2016 north_america_map.img) from the internet source.
And I pasted it into a sd card which already formated( I renamed the .img file as gmapsupp.img as someone instructed. He instructed to create a new folder as Garmin in a sd card and paste the .img file into Garmin folder)----> all the above process were done in my pc.
And I inserted the sd card into a sd slot of Garmin device and rebooted the device but nothing has been changed.

I think I did not do right on that. Can you tell me what was wrong?
I am very curious about how the device works with sd card because the internal memory in the device and sd card are the different drive.

*sd card size: 16gb, formatted,
The device has a sd slot but a sd card didn't come with the device at the time of purchase.