Thanks for that, I will look in main.c. The official BlackPill images will run on application of power. Another distro I am trying to work with (STM32H723VGT6) runs from the REPL, but not on application of power with no USB connection to a PC.

main.c line 141 has:
#if MICROPY_HW_FLASH_MOUNT_AT_BOOT
Where is that defined?

in mpconfigboard_common.h there is #define MICROPY_HW_FLASH_MOUNT_AT_BOOT (MICROPY_HW_ENABLE_STORAGE), and further there is

#if defined(MICROPY_HW_BDEV_IOCTL)
#define MICROPY_HW_ENABLE_STORAGE (1)
#else
#define MICROPY_HW_ENABLE_STORAGE (0)
#endif

and then later on there are defines for MICROPY_HW_BDEV_IOCTL dependent on the type for flash internal/external, which is port dependent. But it is not related to USB.

Thanks for further explanation.

dave@davef:~/micropython/ports/stm32$ grep -r "MICROPY_HW_FLASH_MOUNT_AT_BOOT"

main.c:#if MICROPY_HW_FLASH_MOUNT_AT_BOOT
main.c:    #if MICROPY_HW_FLASH_MOUNT_AT_BOOT
mpconfigboard_common.h:#ifndef MICROPY_HW_FLASH_MOUNT_AT_BOOT
mpconfigboard_common.h:#define MICROPY_HW_FLASH_MOUNT_AT_BOOT (MICROPY_HW_ENABLE_STORAGE)

Somewhere, MICROPY_HW_FLASH_MOUNT_AT_BOOT is made 1 or true ... but I can't find it.