Hi there. I had this same problem as of yesterday. I’m using a Cannon Z100 as a deck and have a new imac duo. I ‘fixed’ the problem by switching firewire ports. Not sure that I like that ‘fix’ (as it implies there is something wrong with the other port and the machine is new) but that’s what I did and it works now.
I’ve been reading up on this and see this problem posted quite a bit, although not a lot of solutions. One other I did see was to reinstall Quicktime, but you need to trash some library files first.
Good luck – frustrating problem. Hope you got it solved already!
K