My Etrex 20 developed a display problem that meant the screen was a silvery grey background when navigating a track route that I downloaded. This made it almost impossible to see the track when on my bike handlebar. Garmin could not help me even though I sent screenshots of issue..
I then put a 16GB SD card in the device, connected to Grmin Express and downloaded maps as instructed. The screen now works perfectly. A result !