i have not booted from the HDD, but i have instead booted from an SD and stored all my roms on the HDD.
IIRC there is an aussie on youtube that did the direct boot rom from HDD, with no SD, but unfortunately ive forgotten his name.
if you want to boot from SD and run HDD for rom storage, MLP has a video on this as well, theres also a cpl threads on here already.
ill try and dig up that other vid for you.