Make sure you are using the right partition information in all places. I don't know how the internal flash is numbered on the 810 but I boot from external flash on n800. I have one partition on the device.

For this use /dev/mmcblk1p1

You must also make sure your boot menu is set up to use the same one as well.

The error message is showing that you likely did not point the boot menu to the right device.