Disconnect the FireWire drive competely and try capturing a minute or two to the internal drive. If you don't have room for that, then that's the problem. The System Drive needs to have good elbow room for live video in either direction.
If it succeeds, you may have bad interaction between the two FireWire devices.
There can be problems with the wrong versions of OSX, QuickTime, and Final Cut. What are they?
Let us know.
Koz