Putting a hard drive on the same FW bus as a Duende is typically not a good idea.
Duet and Duende play nicely together one a single bus.
Don't know if this
helps, but it is at least a practical solution. All depends on if your Mac has 2 FW buses
or a split bus running to the 2 ports (which I'm afraid my expertise doesn't run to).
You could alwys fit one of the 7200rpm drives into the MacBook and use USB drives
Head of Business, Solid State Logic.