Well, the only mikes I have are what I plug into the Saffire, so it is recording from them. Is that what you mean by "external mic"?
Do you not want to record from a mic?
Something else... I don't have a sound card in the computer anymore.
A few years ago I was using a SoundBlaster Extigy as an interface and I had constant conflicts with the stock RealTek, so I removed the RealTek HD Audio program from the computer. No conflicts after that, but the Saffire is now the only sound device I have.

Hey Greg
   I did not want to record the mic.  Just the audio straight from the soundcard, so you can hear the true mix coming from the speakers, as opposed to the mix coming through a microphone.  I record the dialog in Sonar and add them together later, in Vegas.

    It's actually a bit of work (mainly because I'm still learning how to put together the video in the video program.  I ended up routing from my MAudio out to my Reatek 'in'.  That's weird you were having conflicts with the two sound cards.  I've always had 2, one for my 5.1 surround and the other for recording applications.  Have not run into that particular problem yet.../knocks on wood

 
I've never done that.
I'll give it a try the next time I have the DAW working. I may end up with the same problem you're having.