It's not enough to format to FAT32 with Garmin name. I'm serious, FAT32 filesystems are different, and sometimes Garmin devices don't understand FAT32 filesystem. Use RMPREPUSB as suggested here: [Only registered and activated users can see links. ]
GarminCure3 can't break the pre-boot mode on nuvi 12xx, as it doesn't patch boot.bin, so you're doing something wrong.
Try holding the power button for ~30 seconds to turn off the device completely.
Also try pressing the LCD as close to the upper left corner as possible.