I'd suggest using only 1 firewire device.. and that would be your audiointerface..
most of the external firewire hd's have also an usb-connector.. take that one instead..
it's not suggested to use another firewire device anyway if you already use it for an audiointerface!
I've never seen a laptop with 2 firewire connections neither.. not even a mainboard..
so if you really really need a second firewire connector you'd have to buy a pcmci-firewire-card..