I found an older thread related to this topic, but it was locked and had no responses from Garmin, so here goes...
I was loading a custom map ( which I've done successfully before: google earth image overlay -> .kmz saved to custom maps folder) into my work GPSMap 62STC, safely ejected the unit from the computer, picked it up and pressed the power button ... It gets through the copyright screen and into the "loading tracks, waypoints and routes..." screen, then after about 20 seconds just shuts off. Plugging it into the comp via USB, the unit just repeats the problem, gets to the "Loading yadda yadda..." screen and just shuts off, as if it doesn't recognize its being plugged in via USB and tries top boot up normally.
I tried swapping the batteries - no help
I tried multiple master resets - no help (twice it started to reboot and just showed a screen with some broken pixellated lines of blue and black)
I tried booting it using force mass storage device (holding an arrow key while booting) - the computer (windows 7 machine) makes the noise when an external device is plugged in, the unit just sits at the first "Garmin" boot up screen, and about 30 seconds later the computer makes the noise of removing an external device, and the unit goes through its problematic boot up -> shut down routine...
