The only files you should delete are unused voice files (.vpm), pictures files (.jpg), sum files (.sum) and you can delete the gupdate.gcd file as it is not used once the unit has been updated. You can also delete archived gpx files (.gpx) within the gpx archive folder but do not delete the current.gpx file (this file holds your favorites) or the gpx folder. Deleting folders is not recommended. That is a good way to delete files that you may need.
You can also right click on the Nuvi's drive letter and select "tools" and run an error check on the flash drive to make sure there aren't any errors on it that may involve how much space is really free.
Lastly, you may have to limit the map region, so that the map isn't too large. For example, instead of having US & Canada on the map, perhaps only have the US with the lower 49 states. You can limit it even further by selecting lower 49 states and selecting regions like Southwest (SW) or northwest (NW) etc.
f you really want to bypass the whole memory issue, just put the map on an external micro 4GB SD card. This way, you don't have to worry about deleting anything off of the flash drive.
I hope this info helps.