Hm, are you sure your device has a mass storage mode? I think it doesn't use files at all and maps are sent to it in a different way using WebUpdater or some other software.
I think so because of the following reasons:
1) This firmware has "GPSmap 276C" string inside. As far as I know GPSmap 276C doesn't have a mass storage mode and maps are stored inside dedicated flash regions.
2) This firmware doesn't have "gmapsupp" and ".img" strings inside.
If I'm right, then GarminCure3 isn't suitable for your device. What you need is erasing a few flash regions, for example, the region containing the map and the NV memory region.

BTW, what were you doing when your device failed?