I have never had consistently good results from ‘daisy-chaining’ firewire connection to capture video. You might try swapping the devices’ position in the daisy-chain to see if that fixes the problem.
When you capture DV, you are capturing a data stream at the rate of 3.75 megabytes per second. Design-wise, firewire400 transfers data at 50 megabytes per second, so you would think it could easily handle two 3.75mbs streams. (I’m doing this from memory, so don’t quote me on the exact speeds.) Firewire is sort of a bus-protocol in that all the devices see and talk to all other devices.
However, while 50mbs is the maximum data transfer rate, fw is also controlling the drive and the deck. The hard-drive inside the case is nowhere near as fast as the fw connection. The deck may have all sorts of wait-states to accomodate fast-forward/rewind/shuttle/etc. Also, firewire is a very “chatty” protocol and each device spends a lot of time talking to each other device.
When I had a similar problem, I finally added another fw card with multiple ports to my PC. However, you can find fw hubs from B&H (and probably Amazon) for less than $100.
Hopefully some other folks here have had specific experience with fw hubs and Macs.
Jim