Per this thread
http://my5cent.spdns.de/en/banana-pi/banana-pi-bananian-von-festplatte-booten.html
Mount the boot partition and edit the startup params:
mount /dev/mmcblk0p1 /mnt/
pico /mnt/uEnv.txt
Change the parameters
from:
root = /dev/mmcblk0p2
to (or what you like):
root = /dev/sda1
