I also stumbled upon this issue when trying to use bcache as my root filesystem (on Ubuntu 13.04). Through trial and error, reading these posts and the bcache docs, I was able to fix by stopping the bcache device during the reboot/shutdown process. I created an init script /etc/rc6.d/S59fixumountroot that contains:
#!/bin/sh
echo "Stopping bcache device"
echo 1 > /sys/block/bcache0/bcache/stop
echo 1 > /sys/block/bcache0/bcache/cache/stop
That did the trick! :)