firstly you must enable hidden folders in windows:
When you start windows explorer ,click tools,folder options, select the view tab then put the dot in 'show hidden files and folders' also make sure there is a check mark in 'display contents of system folders' -
Then create a folder in the root of the nuvi drive called "Map" (no quotes) and copy your desired map into that folder. The map does NOT have to be called gmapsupp.img nor any of the older nuvi naming conventions...in this case you should give it a more descriptive name such as Topomap.img (or anything you want).It must end with the .img extension.
The trick works on SD card too