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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