DId you get a response and fix for this - I have the same problem now
Check that the first 15 characters in the new course name doesn’t match a course name already on the device.
If there are lots of courses and activities stored on the device, try deleting some. I recall that there are limits on the numbers of courses and activities that can be stored even if space is available.
Thanks, Peter. I did find that detail in past discussions and tried it without success. I resolved it in the end. I think my NewFiles folder had somehow corrupted. I deleted both the NewFiles folder and the Courses folder - these were re-created as empty folders when the Garmin Edge restarted and then when I copied a gpx file to NewFiles everything worked as expected . Phew !