I'd avoid using dd to test. I did this when I first built the kernel and the device rebooted after a short while. I first thought this was a problem with the patch but to be sure flashed the original rootfs back to the device and tested again and still got the reboot. I've reported it as a bug.

I was doing

dd if=/dev/zero of=/media/mmc2/big bs=1M count=100

Instead, I've used the mass storage driver to copy stuff onto it.