Thanks GTmech.
  In line with the quote from Noel there: when I ignore the message, hit No, there is no audio, so a disconnect has deffo occurred; my firewire device is, power-wise, "on" but registering LEDs that would indicate a mammoth increase and peak in gain; on opening windows audio, all refs and icons for my firewire device have disappeared, despite the device being "on"(connected, powered, lit up). There thus would have appeared either a firewire driver failure...but whether triggered by X3, Windows or my firewire drivers, I've no idea. My own frequency of this issue increases with frequent scrolling through Dim Pro. When I've then closed X3, then turned my fw device off then on, then fired up X3(which I always do as admin by default), all is dandy until (almost) randomly it ain't.
  (I hope I don't give the impression I'm on a hijack here...if I were seeking personal help I'd post a thread; just throwing in observations that may(or not) assist the OP and any troubleshooting process.)

well, PreSonus StudioOne at least went about an hour last night before I decided to hit the rack.  I came down this AM though and the transport was stopped so at some point RME threw up.  No error messages though with the PreSonus app so no telling what happened - however, I can attest at this point that my system is pretty screwed up.  
 
I went over to simply restart the transport on StudioOne and got a Windows error popup titled "RME FireFace Firmware Mismatch".  The error says:
 
This driver version requires at least firmware revision 2.77.  Please download the latest flash update tool from RME's website to update your FireFace.
 
Um, I have 2.77 "Hardware Version" and the very latest FF800 driver off their site.  I saw this a couple times randomly with Sonar too when I was having audio error problems.
 
Anyway, my system is kinda hosed now.  Without rebooting I tried to restart StudioOne and got a hardware error on their startup screen (kinda scrolls through what's loading/going on...different than Sonar) and even after rebooting everytime I go into StudioOne nothing works and I get that RME Firmware error popping up all the time.
 
Sadly, even when I go into Sonar, the transport plays but I can't hear anything...the lights on my UA 2192 aren't coming on (output meter) so even though Sonar says there's activity on the master fader pushing to the RME, nothing is making it via ADAT over to my 2192.
 
*SIGH*
 
So, seems to me like RME has some work to do as well.
 
I don't know - for those of you that have systems that work properly can you keep a project looping for hours and hours on end without the transport stopping?  Just trying to determine if my 'test' holds any water.

 I can loop for a while, but then midi and audio lose sync..  ;)

I have a very simple project - one track with me talking into a mic for about 30 seconds...no FX...and then just looping it.  So, it's not hurting anything - barely even get a blip on any resource usage, etc...
 
I had never tried looping something for hours before even when things were working well so no idea what the expected behavior might be.  But I do know in this case that something went haywire after letting it run for quite some time...I'm going to have to try to uninstall/reinstall RME drivers or something - nothing works at this point and of course the RME drivers in the tray show that everything is working great.  RME Mixer shows signal, etc... but nothing works in the DAW's.  UGH.

All SONAR is doing is reporting the message that a device was inserted or removed. This is not new - this feature was added way back in SONAR 8.5. See the topic Change I/O devices without restarting SONAR in the notes. So either some operation is causing the device to really disconnect or the message is being sent at the wrong time.
If ignoring the message causes lost audio to the device then its really disconnected somehow.

Noel Borthwick [Cakewalk]
All SONAR is doing is reporting the message that a device was inserted or removed. This is not new - this feature was added way back in SONAR 8.5. See the topic Change I/O devices without restarting SONAR in the notes. So either some operation is causing the device to really disconnect or the message is being sent at the wrong time.
If ignoring the message causes lost audio to the device then its really disconnected somehow.

I remember getting this msg and reporting it with my FF800 quite a while ago. It may have even been in the 8.5 cycle. This still persists with my UFX and brand new cables, motherboard and all. 
 
There are a few of us that have not really had a disconnect Noel. How would Cakewalk prefer to proceed with the msg being sent at the wrong time or if it should have been sent at all? Or is this an issue to pursue for Cakewalk? It is an annoyance, but not a showstopper for me since replying no basically clears the msg and audio continues.
It is clear we are talking about someone who does likely have an issue with their system(OP) as well as the other issue where the msg may not be issued correctly.

It is clear we are talking about someone who does likely have an issue with their system(OP) 

How do I have an issue with my 'system'?  I'm using a high end MSI motherboard, the same Syda PCI 1394 card I've been using since Sonar 8.x days (with no problems) and now all of a sudden with X3 I'm getting the same random disconnects with RME products that other people are seeing as well?
 
All I know is that I've been sitting here listening to a loop run in PreSonus StudioOne for almost 2 hours with no disconnect problems whatsoever.  If I switch over to Sonar and run a similar loop - I'll get that audio error in less than 10 minutes. Guaranteed.
 
So, maybe RME's got some problems too - but clearly Sonar does as well.  There's absolutely no reason I can think of where you tell SONAR to "NOT SHARE" the interface and to randomly get disconnects like that.
 
I also can switch my RME to the primary sound card and can listen to music for hours on end without any disconnects. 
 
As I mentioned previously, the error that SONAR shows is exactly the same one that shows up if I phyiscally remove FW cables.  I'm not saying that's what it 'thinks' is happening but effectively, that's the symptom.
 
Either way, something wrong...and there are many people on here telling you so.  Hopefully someone will either attempt to reproduce or somehow look more at what we have going on to see what the problem is and how to remediate it.

Ken, list your recipe for the SONAR looping. As long as it is documented, we can all follow it the same way. Do you have a large region looping? how many measures? Is it midi and audio? Any FX? Did I miss something? I have time and will try to help get it repro'd.

I open a blank project, create 1 track, talk into a mic with it recording for about 30 seconds and set it to loop.  I set the master to my UA 2192 output (it's connected to the RME via ADAT) so I can hear it in the speakers and let it go.  No FX, no nothing else - just as vanilla as possible.
 
I've been sitting here listening to a loop I created in StudioOne at 10:22am CST (it's now almost noon so it's been running over an hour and a half with no issues) using a similar setup.
 
As I said, I guarantee I can switch back over to Sonar and do the same thing and there's no way it will go for this long - under 10-15 minutes tops it will die with that audio/RME error.

I started a loop of 8 measures at 12:05 with my UFX. This has midi, audio and FX in it. I will let it run for 30 mins or so. If no issue, I will get a vanilla setup like you have and give it a go.
 
btw, what state are you in? I am also in CST.