Just discovered that there is a "hidden" built-in Diagnostic Utility in Garmin Nuvi 255w GPS navigator.
You can access it by tapping on the battery icon on the upper right corner of the screen and hold for more than 5 seconds until the Diagnostic Utility come up. Here is how it looks like:
This built-in Diagnostic Utility allows you to see detail technical information of the GPS navigator and the attached SD card (if any). You can also use it to perform some test on its memory, voice, display, and other functions.