Zumo 660 delete route
Just to be sure about the answer, I will answer how to delete a route, which you have deleted form the list on the GPS, under "User defined rotes" and delete. The route is still on the GPS, as it is most likely stores as a file, which you need e.g. Windows Explore to delete.
You need to connect your GPS to the PC via the USB cable, and then you will find the data under:G:\Garmin\GPX (assuming the Garmin is named "G" drive). Here are the gpx files as well as "Current".
If you delete "current" you have removed them from the GPS, but, they are still to find in your folder "routes" or "User defined routes". Then just delete then here also, and you have a clean GPS.
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Just to let you know, that you can create a free profile on
www.tourstart.org where you can store all your routes, so you can always later get transfer them direct to your GPS.