For newer kernel, the LED is configured as heart beat device. You can not control it. You can control it with led trigger.
cat /sys/class/leds/user-led2/trigger
none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock mmc0 mmc1 timer oneshot [heartbeat] backlight gpio cpu0 cpu1 cpu2 cpu3 cpu4 cpu5 default-on rfkill0 mmc2 rfkill1 rfkill2
For example, you can set it to emmc read/write status with following:
echo mmc1 > /sys/class/leds/user-led2/trigger