The recent generation iMac's do not have a Firewire 400 port. They have a Firewire 800 port instead, which uses a 9-pin connector. You would need a 9-pin to 6-pin cable, also known as a "bilingual" cable. Granite Digital makes some good ones, although they are a bit more expensive than run-of-the-mill cables.
Firewire 800 is backwards compatible with Firewire 400 devices except for the cable type.
Regards,
-- Al
Firewire 800 is backwards compatible with Firewire 400 devices except for the cable type.
Regards,
-- Al