When Garmin created Mapsource a decade ago, Apple had a small and declining share of the PC market. In that context, Garmin chose to use the Windows registry to manage the maps. Since obviously a Mac does not have a "Windows registry", existing maps products cannot be installed and run on a Mac. To better explain, I'll use TopoDemo to illustrate the map install problem.

For a PC, Mapsource uses the registry to find the map segments (8-digit.img), display a thumbnail of each (TopoDemo.img) and link between the thumbnail and the segments (TopoDemo.TDB). Therefore installing the maps in a different OS is a big problem.

When you send a group of map segments to your GPS, Mapsource builds a text lookup table and embeds the img files into a single gmapsupp.img file. You can rename any *.img file gmapsupp.img and your GPS will show the map. Look in the folder Garmin on your SD card, its in there.

My suggestion to Garmin users that own a Mac, buy a PC, they're cheap, and use it for these PC only tasks. All you need is the box with XP, you should be able to use your existing monitor and USB KB and mouse. People are paying $500-600 for a Colorado w/o batting an eye. With a little hunting, a new PC with Vista can be had for $200 and a used one for half that.