For anyone interested:
I just noticed with the pairing of my maudio fw410 and my lacie external hd (firewire also), that I got strange clicks and pops ONLY when I played guitar over parts of a piece that were being read (I suppose) from the external hd where all my data is stored. In other words, no pops in otherwise unrecorded spaces on the track. I tried the demo tracks that come with ableton (and are on my main hd), and had no problem with clicks while playing guitar over other tracks. I moved all my files over to the internal with the same track etc. and now no pops. I can only guess that my two firewire ports are sharing one input or are somehow competing for bandwidth? Not so much advice as a potential source of trouble - I still have no idea if the explanation follows the routing of the firewire ports, but now no problem.