The information below regarding importing custom routes was copied from Zumo Forums. This is good information, especially for those with not a lot of experience importing custom routes. Pay particular attention to the paragraph entitled Bottom Line.
Route Import:
"Import" refers to making a route active. Internally, the route is transferred to the Zumo's internal memory and placed in the active list. It should occur automatically when a route is transferred from MapSource to the Zumo (See MapSource Transfer below) and the Zumo is restarted; however, there are times when this doesn't happen. You can also manually import a route by selecting "Tools/My Data/Import Data/Routes".
MapSource Transfer:
"Transfer" refers to using MapSource's Transfer Menu to move waypoints and routes to the Zumo or SD card. You may also directly save route .gpx files to the Zumo using Windows cut and paste or MapSource's "Save As" menu. The transfer rules are:
1. If there are less than 20 active routes, new routes transferred to the Zumo are temporarily saved in the "Temp.gpx" file, are immediately imported, made active, appear in the "Custom Routes" list, and no recalculation message is displayed. My tests indicate that there is no recalculation on reboot and the route is exactly the same as on MapSource!
2. If there are already 20 active routes, new routes transferred to the Zumo are temporarily saved in the "Temp.gpx" file, but are not imported nor made active and will not appear in the "Custom Routes" list. You won't receive a reminder message asking you to import the routes after restarting the Zumo so you have to remember to manually import the routes after deleting some active routes. This always results in a lengthy recalculation regardless of the recalculation preference setting in the "Tools/Settings/Navigation" section. (It sometimes seems to lock up my Zumo, but all is well after a restart...major bug!)
3. If you transfer new routes to the SD card, the routes are saved on the card in the "userdata.gpx" file, are not automatically imported nor made active, and you will have to import them regardless of the number of currently active routes. You should get a message within 30 secs of Zumo startup which asks to import the routes or you can manually import the routes later. This always results in a lengthy recalculation which can take several minutes depending upon the number of routes being transferred and their complexity. Again, the Zumo sometimes locks up, but is OK after a restart. Unless you are sharing routes with another person or running out of Zumo drive space, there is no advantage of transferring routes to the SD card versus the Zumo.
4. You can also save a route's .gpx file directly to the Zumo's "garmin/GPX" directory or the SD card's "garmin/GPX" directory via Windows cut and paste or use MapSource's "Save As" menu and the Zumo will recognize the route. These route files are permanently saved until deleted via Windows so they are very safe as MapSource transfers don't affect them. The route/s must be imported and will always be recalculated. Typically, import will also lock up the Zumo, but is OK after a restart.
Bottom Line: The only way possibly to avoid recalculation is to use MapSource's transfer function to move the route to the Zumo. Before doing so, make sure there are less than 20 routes in the active list. All other options require an import which always results in a recalculation and possible different routing.
MapSource Transfer File Name Feature/Bug:
MapSource transfer will NOT replace an existing active route of the same name (first 36 letters significant) even though MapSource says the transfer was OK. This creates a problem when you modify an existing route with MapSource and transfer it with the same route name. The new route is saved, but not imported nor made active. There is no warning that your new route wasn't made active or that the old route remains active and unchanged. Your options are:
1) Delete the active route from the Zumo prior to transfer OR
2) Change the route name in MapSource or on the Zumo prior to transfer OR
3) Manually import the route even if you saved it to the Zumo drive. The route is recalculated and made active with a slightly different name by appending a number to the end.