In the “full main.cpp” I gave you, dumpSdPins() did this:
pinMode(SCK, INPUT_PULLUP);
pinMode(MISO, INPUT_PULLUP);
pinMode(MOSI, INPUT_PULLUP);
…and you were calling dumpSdPins("after-idle-clocks") inside tryMountWithBus(), after bus.begin() and the 0xFF idle clocks, but before SD.begin().
That means: right before SD.begin(), you were accidentally turning the SPI pins back into inputs. The card then can’t respond, so you get endless:
sdCommand(): Card Failed! cmd: 0x00
f_mount failed: (3) The physical drive cannot work
That matches your new log perfectly.
