I've just discovered another hidden feature in my Garmin Nuvi 255w GPS, which is the system debug info screen. Here is the trick to get there.
In the map screen, touch the left capsule beside the Menu button to open the Trip Computer screen. You'll be able to see a speedometer on the screen. Press and hold your finger at the center of the speedometer for at least 6 seconds, and you'll be able to see the system debug info screen.
All the information shown in the few pages of the system debug info screen are very technical. Interestingly, you'll be able to find out that the Garmin Nuvi GPS navigator can automatically defrag its storage from time to time.
You can just press the Quit button to quit the system debug info screen.