Ahhh... great point guys. I was not clear about UPDATING from the SD. I phrased like you can just moved the card and that puts the firmware on the new board... not true.
So, to re-summarize: (everybody probably gets this... but just in case... )
config.g is loaded from SD every time a board power cycles or otherwise boots.
config-override.g is loaded from SD every time an M501 is executed, and therefore would be loaded at power cycle and/or boot ONLY if the config.g contains an M501 (preferably somewhere near the end)
Other "whatever.g" files are loaded from SD as various events occur, like tool change, or print cancel, or...
Therefore, swapping an SD around different boards effectively "instantly updates" all the various ____.g files on the SD.
Duet/Reprap Firmware is on the SD. However, firmware RUNS (executes) from Processor flash.
Similarly, WiFi Firmware is on the SD but runs from flash in the WiFi module.
Updating Processor and/or WiFi flash can be accomplished via:
A) Uploading a properly named file via Duet Web Control (and clicking "yes" if prompted)
B) Placing a properly named file in the /sys directory of the SD card, and then issuing specific M commands.
C) Using a combination of USB, SAM-BA, and hardware reset/erase buttons.
The required names and exact steps are detailed in the various "Fallback procedures".
