The statement kind of stands for itself: "Once the flash starts properly the device will hold in preboot until it finishes" - in other words preboot won't drop while there is a proper connection with data transfer between Updater.exe and the device. A flash is generally considered to have started properly when "Loader" shows on the screen however that might be not considered conclusive in your device's case of course. Certainly if you see "Software Loading" after some seconds it's flashing correctly and you can safely release the power button however it's fine to hold it until updater.exe reports success. This info is now likely moot anyway because you've been unable to flash it 'conventionally' in preboot.

Your lack of result with the card commands wasn't totally unexpected and provided the device is physically healthy, your assumption is likely correct in that the boot process is not getting far enough along to read from the card as it's being relegated by the attempt to flash something from the internal device memory first. Time to try using a modified boot.bin/Ldr.bin as an RGN, this is an attempt to get the card read before stuff is loaded as part of the 'normal' boot process and if it doesn't work i have no further suggestions and would strongly suspect physical flash chip damage. You'll need a hex editor for this, i mostly use XVI32 which is freeware however any hex editor will do. Here's the procedure:
  • Go the Ldr.bin file which you find in the 1364 folder and copy or move it somewhere onto your PC. REMOVE the Ldr.bin file from the card afterwards if you only copy it.
  • Look at this Post #33: [Only registered and activated users can see links. ].
  • Make another copy of Ldr.bin to work with, keeping the original. In a hex editor open that copy of Ldr.bin (it's the boot.bin, which is actually the ramloader ... what? It's Garmin ...), locate and make the change in the 4-byte string from '0C 80 A0 E1' to '0C 80 A0 E3' then save in a separate location.
  • Open the modified Ldr.bin in RGN_Tool and press the "RGN" button, save it as 136401000560.rgn somewhere convenient.
  • Go back to the 1364 folder on your sd card and make sure the only file there is 'Update.txt' before proceeding.
  • Put the card into the aera, making sure it's fully off.
  • Open the 136401000560.rgn file in Updater.exe, start the aera in preboot and try to flash.

Good luck! Hopefully this time you'll have a file 154.bin in the card's root and/or a file update.log in the 1364 folder.