By default, even if you have no space for it, the Map Update program refuses to acknowledge if you have an SD card in the Garmin device (in the 650 and 200w I had access too, anyway). I found the source of my answer in a forum post.
Three files get installed: GMAPPROM.gma, gmapprom.img, and GMAPprom.unl.
To get the map installed, copy all the data from the Garmin device to the SD card, then run the update program. It will think your SD card is the Garmin. Make sure to make a backup, unless you want to go through the pain of copying 1.5 gigs of data over USB 1.0 each time you need to do this.
Once the map is installed, delete everything else and rename the key files: GMAPSUPP.gma, GMAPSUPP.unl, and gmapsupp.img.
Eject the card, plug it into the Garmin, and boot it up. In the system prefs, map, map info page you should see the latest map file (2011 NT when I got this to work).
Hopefully, Garmin will eventually fix this…