Windows 7 Pro SP1
Express 3.1.7.0
I plugged in my Nuvi 1390, fired up Express and was told there was a new version of CN NA available (which was wrong, but I wasn't paying enough attention).
When I tried to download and install, I was told that there wasn't enough space on the device and that I could fix this by installing an SD card.
Problem is, I already had a 4GB SD card in place.
By removing the existing map image from the SD card, Express was then convinced it could fit the new map onto the device.
Apparently, Express isn't able to figure out when it's replacing an existing map set and to include the size of the old map image into its available space calculation.