I opened a support ticket for this but fear I may be persona non gratis as the crickets are deafening.  So I'd be interested if anyone else has experienced this and (hopefully) found a resolution.  After years of technical challenges I had finally gotten my studio functioning like a toaster --- turn it on and go.  Sure, every now and then I'd get a crash that required a restart but light-years better than the old days.  One essential piece of gear in my studio is a Mackie MCU Pro.  Changed my life.  Always worked like a charm and I thought it bullet proof.  Until...
 
I had a crash when opening one of my tracks (when I have rare crashes that's nearly always when it occurs).  A shared dll issue of some sort so said the pop-up.  But afterwards my MCU was not working.  Typically a restart brings everything back but on this occasion I didn't hear that comforting sound of the faders positioning themselves.  I assumed the worse.  Cables - check.  Listed in Sonar devices - check.  Listed in Device Manager - check.  The MCU has no drivers so what next?  I contacted Mackie and Cakewalk and heard back from Mackie the next day.  They assured me if Device Manager sees it, everything is ok with the hardware.  No response from Cake but I was sure they were busily working on it.
 
The MCU is uber-important to how I work so I soldiered on looking for the issue.  USB cable - double check.  I downloaded a USB monitor program and lo-and-behold the MCU was talking and the PC was listening.  It was Sonar.  I shared this with Cakewalk.  I unchecked the MCU from devices in Sonar and did a couple restarts, re-checked them and Yay!  The MCU was back...  for a short while.  And then gone again.  While Cakewalk Support formulates their response to me, has anyone else faced this?  Why is Sonar dropping the MCU connection even if it's listed and functioning correctly as far as Windows is concerned?
 

Are you plugged into a USB 2 or a USB 3 port?
 

riojazz
Are you plugged into a USB 2 or a USB 3 port?
 

Doesn't Mackie MCU use Midi ?
I use Mackie MCU but I run it through midi.

I understand you can run this via MIDI, but he said he double-checked his USB cable.  Although I do not have this unit, I do have a Behringer X-Touch that emulates the Mackie Control Unit.  It is finicky with USB 3. 
 

Do you have other MIDI devices? For some reason, recent Sonar versions tend to mess with MIDI devices once some of them are changed (disconnected, not functional, etc.). Fast indication the device is "lost" is no MIDI activity in the Windows tray (Sonar MIDI icon).

Hi guys... thanks for the replies.  The MCU is intended to be connected via USB and mine has always been connected that way and has always lived in the same port --- which is a USB 2 port, BTW.  You CAN connect it via MIDI but that's not as efficient.  And again, until very recently, it has worked flawlessly --- I mean for a couple of years.  I've gotten it to work again but then the next time I go into Sonar it won't work again.  It's listed and a USB monitor shows that it is functioning correctly.  I have to believe this is a Sonar issue but Cake is silent.
 

I have a MCU pro and it works well with azslow3's AZ Controller and Sonar Plt. There was a Sonar update 4-5 months? ago after which Sonar routinely played roulette with the midi controlllers and the assigned in and out ports. That has been fixed. Occasionally if I don't turn something on when starting Sonar , the in and out port assignments get shifted. So these assignments might be something to check if the Pro isn't working.

As Sonar doesn't allow listing midi controllers using AZ Controller under common names, I also have to be careful in the AZ Controller's preset window to have the right preset selected. One thing I like about AZ Controller is that I can easily see what commands the Controller sends to Sonar. Confirms that I have the right preset and assignments for the right Controller.

I also noticed that a couple of Win10 updates messed with a couple of audio drivers necessitating their reinstallation. This wasn't with the MCU directly, but did mess up what Sonar was detecting on start up.

It might be worth checking if another MIDI aware program (other than Sonar) can actually open the USB MIDI ports. If it can't, then try re-installing your USB driver.
 
I've also become victim to the MIDI port order issue, although in my case I noticed it by my MIDI keyboard not working (the control surface was using the keyboard's MIDI device rather than the MCU's), so it's best to check this isn't happening. When this happens, I've found you have to remove the control surface within Sonar and add it again, as just changing the device back to what it should be didn't work for me.
 
Incidently, the reason my MIDI ports were in a different order was due to me plugging in the MIDI interface after I'd booted up. When doing a clean boot of Windows with everything plugged in, I've not had any MIDI port order issues.
 
Another thing to check perhaps is your power settings - you should disable any powering-off of USB devices.

If things aren't powered-on PRIOR to starting Sonar all sorts of unwelcome issues can arise.  I learned that lesson long ago.  My system hardware has been unchanged for quite some time.  The only variables really are Sonar updates and the dreaded Win10 updates.  Win10 routinely dumps my RME driver when it updates requiring reinstalls.  Not a big deal but a PIA.
 

jtma508
If things aren't powered-on PRIOR to starting Sonar all sorts of unwelcome issues can arise.  I learned that lesson long ago.  My system hardware has been unchanged for quite some time.  The only variables really are Sonar updates and the dreaded Win10 updates.  Win10 routinely dumps my RME driver when it updates requiring reinstalls.  Not a big deal but a PIA.
 

As a PSA, I can tell you how to stop the dreaded windows 10 Corrupdates.
 
Search, control.
In Control panel select administrative tools.
In Administrative tools, select services.
In Services find Windows Update.
Right click the Windows Update service and select properties.
Change the Windows Update Service's Startup Type to Disabled
Restart Windows.
 
:-)