I think I've found the solution to the problem 
http://www.synchrotech.com/product-usb/usb20_to_usb20_expresscards_host_adapter.html
http://www.lindy.co.uk/firewire-card-2-port-expresscard-34/51500.html
Both are Mac compatiable. I haven't tested it, but they should work!
You can get firewire 800 card but I think USB only supports 480mps.
http://www.synchrotech.com/product-usb/usb20_to_usb20_expresscards_host_adapter.html
http://www.lindy.co.uk/firewire-card-2-port-expresscard-34/51500.html
Both are Mac compatiable. I haven't tested it, but they should work!
You can get firewire 800 card but I think USB only supports 480mps.