There is no such animal as firewire 600. any chance you mean 6 pin, which would be firewire 400? You are a little light on details of what you have where, so I will guess that the mac has a 6 pin firwire 400 connection, as the mixer has a firewire 800 (9pin) connection. In this instance, you need a firewire cable with 6 pin on one end, and 9 pin on the other. The cable MUST be rated as bilingual for it to work. If you do not see bilingual specifically mentioned on the product information, or product packaging, then find a different cable that does.
