i was just saying IF there is already a port at the back of the computer that's how you would power it. and if the cable is a firewire cable i cant think of any other possible way to power it other than an existing port. so if you dont have one then maybe it isnt possible for you to use it.
unless - as suggested you get a PCI FireWire card and that will power this one in the same way i said (and only way it seems it could work) by plugging this cable you have into the port
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