@Raupe
Please state more clearly why you wish to flash standard firmware in this way before i make an SD card kit. I'm very aware that it is difficult to attain preboot in 2xx9 series and it's a very short time to start a flash when the device is in preboot. However Mass Storage Mode should still be available unless the device is bricked or has a faulty USB connection. If it's bricked then it's unlikely to be getting far enough into the boot process for a microSD card to be recognized. If it's not bricked and has a USB hardware problem then placing the higher version of firmware on a card will update the device anyway, to do that the GCD file must be named only GUPDATE.GCD and placed in a Garmin folder on an otherwise empty card.