I dont own one.. but I dont think you can do this.. I believe you can set up an ext HDD to record to *through* the 7xx, but moving pre-recorded files to an ext. HDD might not be possible.. of course I could be wrong..
edit:.. welp.. after more thinking, can't you copy recorded files to the CF? if so.. there must be an option to copy to EXHDD.. I think the drive must be formatted by the 7xx though..