I recently upgraded to X3 from X2 and I'm having an odd problem that I didn't notice until today.  I'm using X3d Producer 64bit on Windows 7 64bit, with a Focusrite Scarlett 18i20 as my main interface.  I have a Presonus Digimax FS connected via ADAT, and a DBX 386 connected via SPDIF to the 18i20.  I'm using the 18i20 clock, with a BNC connection to the Presonus, then to the DBX, where it is terminated.  Everything is fine until I switch on the DBX preamp, at which time the Mix Control crashes with a message of "No Hardware Found" and the audio engine in Sonar stops, indicating that the drivers have crashed.  If I close Sonar and Mix Control, then reboot the interface, Mix Control still gives the message, and I must restart the computer to get the drivers to load again.  This only happens when using X3.  I checked older projects with X2 and everything works fine.  In X3, using the 18i20 with the Digimax is fine.  Using the 18i20 with the DBX is fine if I use the clock on the DBX and have the 18i20 sync to it.  But when I try to use all three, with the 18i20 clock, it crashes.  I checked all my audio preferences and they are the same as in X2, where there is no problem.  I know I must be missing a setting somewhere, but I can't find it.  I have a support ticket in to Focusrite, but they haven't responded yet.  I uninstalled and reinstalled several versions of the Focusrite software and drivers to no avail.  Has anyone else experienced this and how did you fix it?
Thanks in advance.

I don't know if toggling this might help...
Preferences > Audio > Playback and Recording > Share Drivers with other programs
 
The other setting that might be worth toggling as an effort to fix the issue might be...
Always Stream Audio through FX
on the same tab.
 
These are the things I would try.

I would reset and enable default settings on the mixcontrol menu. Then uninstall and reboot. Finally please download and install the latest version of mix control from Focusrite's site. Then do another reset and apply default settings again. Try that combination.

If that doesn't work have a look at renaming AUD.INI file in Sonar.

Cheers..

Focusrite has a new update. Ver 3.4
I dont know if its only for Saffire or for other interfaces.

Thanks for the helpful replies.  Nothing seems to work completely.  It's strange, but it seems that the SPDIF connection crashes the drivers, but I can't figure out why.  I still haven't heard from Focusrite.  I have the most current beta version installed, which for better or worse has updated the firmware on the interface, and I can't roll back to previous versions.  I did get it to reload the drivers after Sonar is launched, so I can use either the 18i20 with the DBX preamp, or the 18i20 with the Presonus preamp, but not all 3 together without it crashing the drivers.  I went through the aud.ini files of X3 and X2 to see what differences there were and did make some changes to the X3 file.  That enabled me to reload the drivers after X3 launches, but didn't completely fix the problem.

The beta version of focusrite is now a full version 3.4
 

Is it possible the SPIDF connected units are set to different sample rates? Is one correctly set to slave and one correctly set to master?

The only mystery here is that only x3 is crashing. Otherwise you'd be looking at the Focusrite drivers. And this just might be the case. 
I'm a little miffed at them as they seem to be under question with driver releases. When I bought my  scarlett 6i6 I went to the web site and downloaded  1.7.
Like yours this includes the driver, the Mix control software and firmware all in a package. I installed this and when I plugged the Scarlett in, Windows7  told me it was installing the drivers? Then it said there was a problem and my hardware might not work?? Then The scarlett opened a window to say it was updating firmware to new version.
Nothing worked. I spent 2 hours mucking about.  Still dead ,, so sent an e mail to support.
In the meantime I grabbed my wifes Lenovo Laptop and re downloaded the drivers and boom, it worked just fine??
Later that day support replied to tell me they had removed 1.7 from the download site as it had "issues" and posted me a link to the Beta drivers?? 
I checked my download on the laptop, sure enough, it was 1.6??  I just checked yesterday and 1.6 is still there on the support page. That version is known by them to be buggy.. so what's with keeping buggy drivers posted?? Not only that, if you want the ones that work, Beta 1.7,  you'll only get them by contacting customer support. I could not find them on the web site.
Anyhow all is working just fine with the beta drivers, but to me that's a little weirdness.
 
So long and short of it is  "Focusrite MIGHT have questionable drivers" . Or the Mix Control software is buggy. Not sure if it's universal to all Focusrite products. And why do they bundle it together. I would prefer a sperate down load for each of the 3 items.

I suggested renaming aud.INI so you start from fresh. I would not edit the file itself. With 3.3 for me the software is stable as a rock and we've heard about similar results in these forums with other interfaces. Have you fully reset your focusrite to defaults yet?

Regardless your usecase specifically with SPDIF may demonstate what you say about the drivers maybe not far off the mark esp if the beta works. I suspect without this it would have been as stable as a rock otherwise.

The sample rates are set correctly, and I'm using the 18i20 clock as the master, with BNC connections to the Presonus and the DBX.  I had them daisy chained, but I have ordered a T connector and will try running lines to each preamp and terminate them both.  When they were daisy chained, the DBX was the last in the chain and was terminated, but the Presonus wasn't. 
 
I agree about the frustrations with Focusrite bundling everything together.  I wanted to roll back the drivers to see if an earlier version would work, but wasn't able to. They also have not been very quick to answer my support ticket.  I contacted them 5 days ago and still haven't received a reply.