One of the things I learned about over the past month was the existence of the STM32 “DFU” bootloader built into many of their MCUs (I’ve wanted to have something like that for years in the devices I’ve worked with):
This allows programming through the USB port as long as there is a “Boot0” pin which has a jumper to power like how it’s done on the Octopus board:
If you have that capability built into the controller board, you never have to worry about SD Cards again (and, for me, that’s a good thing).
STOP THE PRESSES
Before sending this reply off, I thought to take one last look at the BTT SKR Mini E3 V3 schematic after seeing that the ST32G0 has the DFU Bootloader built in and I realize that you could put the board into DFU mode and program it from a Raspberry Pi or Windows PC.
It looks like U2 (the STM32G0B1RCT6) pin 46 (marked "PA14+BOOT0) on the schematic above is also used as the SWD “SWCLK” line and goes to a pin J11, right beside 3.3V.
So, to put the board into DFU Mode, simply power down your system, jumper pins 3 & 4 of J11, power back up, connect your Raspberry Pi or Window PC and run DFU. When that’s done, power down, remove the jumper, power back up and Robert’s your mother’s brother.