I just found another really good reason why you should follow my advice and load all of your North America maps onto an SD card rather than installing them to your Zumo 550 (but this would actually apply to any zumo).
I realized the other day that I am two map versions behind (I have the nuMaps lifetime maps for my zumo), so I downloaded and installed the updated maps to my PC. But, we have a group ride going on this weekend in the Adirondacks, so I am (normally) hesitant to update maps to my GPS the day before leaving. Having the maps loaded onto an SD card means that I can put the latest version of maps onto one SD card, and also have the last (known good) version of maps loaded onto another one as a fall back.
If I had any problems following the routes with the newest maps, I could easily go back to the earlier maps just by powering the GPS down and switching SD cards (physically easier on a 550 than a 66x). The waypoints, tracklogs, and routes are all loaded into the GPS's internal memory, so you wouldn't lose any of those when switching.
Kule! :thumbsup: