http://support.presonus.com/entries/21234478-9pin-6pin-Firewire-800-400-Cable-Why-is-a-Bilingual-S800-to-S400-FireWire-cable-needed-
The FireStudio interfaces are FireWire 400 devices and they ship with a standard FireWire 400 cable. You'll need a bilingual S800 to S400 FireWire cable to connect a FireStudio to a computer that only has the more recent FireWire 800 ports. If you are using Apple's Thunderbolt to FireWire adapter then you'll need a bilingual cable to connect to its 9-pin port.
The bilingual cable is a single FireWire cable that will have a 6-pin plug on one end and a 9-pin plug on the other end. These can be found at many online retailers including the PreSonus accessories store.
NOTE: There are some cables that are sold as FireWire 800 to 400 cables but they actually include a 9-pin to 6-pin adapter on one end. These adapters are problematic and should not be used. Your computer may not be able to detect the FireStudio interface if you are using an adapter.
LINK TO KNOW GOOD FW800 - FW400 BI-LINGUAL CABLES:
http://store.apple.com/us/product/HA833ZM/A/belkin-firewire-800400-9-pin6-pin-cable-2-m66-ft
http://www.monoprice.com/Product?c_id=103&cp_id=10301&cs_id=1030105&p_id=3542&seq=1&format=2
