How did you format the flash drive? Windows is likely to format it incorrectly, you should do it with RMPREPUSB as suggested in the first post.
CURE firmware can't boot, it can only enter the mass storage mode. You need to flash the ORIGINAL firmware (click the corresponding check box of GarminCure3 program) in pre-boot mode.
To enter the pre-boot mode:
1)disconnect the device from the PC
2)Hold the power button until the device turns off completely
3)Press and hold the upper left corner of the LCD
4)Connect the device to the PC
5)Wait until updater.exe detects the device