I think you dont have enough space on your device.
try this: in a 4gb sd or pendrive create a folder named Garmin . Copy in it your GarminDevice.xlm you find in Nuvi/Garmin .
Insert the sd in your pc and start Mapupdater. I guess your installation will be successful.
After that copy all the files in your Nuvi .
You can put some files in the sd for example:
SD/Map/gmap3d.img
SD/Garmin/JCV/*.JCV
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