Modern devices are notoriously difficult for everyday users to flash with an RGN file in preboot. It's now a real challenge to initiate a flash from Updater.exe. Because combined with the difficulty of first attaining preboot, it's then only held by the device for a brief period. Once a flash is actually initiated however it will hold preboot until Updater finishes flashing. Some older device could be triggered into preboot via the hidden menu behind the battery symbol, but Garmin removed that capability in later series. They really don't want us mere mortals messing with their devices 'under the hood' and limiting preboot availability helps achieve that.

You might be better off flashing individual fw BIN files using the ramloader (i.e. "boot.bin") as Ldr.bin to initialize TXT commands on a microSD card, read this: [Only registered and activated users can see links. ]. If you're not comfortable making an Sd flash kit yourself after reading up, then just ask and i'll make one for you ASAP when i have some spare time.