I've got a similar problem with nüvi 255.
The problem was that the last update in 2010 did the update in gmapsup.img instead of gmapprom.img. So I lost a lot of space with 2 maps. All Traffic was shown, but not routable... But Anyway... i renamed booth files and installed the whole europe- map on the micro-sd-card. The new map sizes 2,2GB, but Nüvi have only 1.4GB space on the internal flash. Now it works perfect.