SD and MMC Device Partitions ============================ Device partitions are additional logical block devices present on the SD/MMC device. As of this writing, MMC boot partitions as supported and exposed as /dev/mmcblkXboot0 and /dev/mmcblkXboot1, where X is the index of the parent /dev/mmcblkX. MMC Boot Partitions =================== Read and write access is provided to the two MMC boot partitions. Due to the sensitive nature of the boot partition contents, which often store a bootloader or bootloader configuration tables crucial to booting the platform, write access is disabled by default to reduce the chance of accidental bricking. To enable write access to /dev/mmcblkXbootY, disable the forced read-only access with: echo 0 > /sys/block/mmcblkXbootY/force_ro To re-enable read-only access: echo 1 > /sys/block/mmcblkXbootY/force_ro