After installation (an upgrade from X2), none of my VST3s showed up, nor did some VST2s.  I tried the "Reset and rescreen" trick recommended by Cakewalk, but that didn't work either.  Worse, Sonar crashed a couple of times in the process, usually when I tried to use the plug-in manager for scanning.  The last time, Sonar not only crashed, but it took down my computer -- everything went black and the machine rebooted.  This is the first time I've seen ANY software mess up so badly that it took Win7 down without a BSD.
 
I'm going to try uninstalling X3, uninstalling X2, doing a manual registry scrub and file delete from User Data and then re-install.
 
I've never had this kind of trouble with a Cakewalk product, and I've been using this DAW since it wasn't yet a DAW and just a sequencer called Cakewalk.

You do not list the spec of your machine, OS or audio interface if you have seen a BSD I would look more to the drivers of the audio interface than Sonar X3.

My TSAR-1R Soft Tube reverb VST3 unit is working perfectly.

karhide
You do not list the spec of your machine, OS or audio interface if you have seen a BSD I would look more to the drivers of the audio interface than Sonar X3.

It's Win 7 on a quad-core 2.6 GHz Intel machine with 8 gb of RAM that I built myself.  The interface is a Fast Track Ultra.  It did not crash because of the interface or the OS, but because of a problem handling VSTs.  It did NOT blue screen -- the final time it took down the whole machine and caused a hard reboot.  X2 (nor any other version of Cakewalk/Sonar) never had any problems so severe as to take down the entire system, particularly in Win7.  
 
The VST3s that are not showing up include Ozone 5 and Celemony, both of which I bought and have been using for a year or more.

try to avoid the scanning of VST 3 just to see ...one VST 3 instance is loking it ...

PTravel

karhide
You do not list the spec of your machine, OS or audio interface if you have seen a BSD I would look more to the drivers of the audio interface than Sonar X3.

It's Win 7 on a quad-core 2.6 GHz Intel machine with 8 gb of RAM that I built myself.  The interface is a Fast Track Ultra.  It did not crash because of the interface or the OS, but because of a problem handling VSTs.  It did NOT blue screen -- the final time it took down the whole machine and caused a hard reboot.  X2 (nor any other version of Cakewalk/Sonar) never had any problems so severe as to take down the entire system, particularly in Win7.  
 
The VST3s that are not showing up include Ozone 5 and Celemony, both of which I bought and have been using for a year or more.

 
The reboot of the machine sounds like a BSD and would depend on the machine settings if you see the blue screen or the machine will just reboot or as you called it a hard reboot.  
 
Did you install the VST3 plugins at the same time as the VST2 plugs and is the folder you installed them in to scanned by Sonar? 

>> everything went black and the machine rebooted.
 
I'm sorry that you are having trouble, but the fact that your machine is rebooting means some driver is blowing up. There is no other way that this can happen, other than faulty RAM or some other hardware failure.
A simple vst scan cannot cause this on its own unless, some of your plugins have copy protection and the driver involved with the copy protection is blowing up. If you look at the event viewer in Windows you should be able to find an event that shows which driver crashed. Once you know that you can troubleshoot it from there.
 
 

karhide

PTravel

karhide
You do not list the spec of your machine, OS or audio interface if you have seen a BSD I would look more to the drivers of the audio interface than Sonar X3.

It's Win 7 on a quad-core 2.6 GHz Intel machine with 8 gb of RAM that I built myself.  The interface is a Fast Track Ultra.  It did not crash because of the interface or the OS, but because of a problem handling VSTs.  It did NOT blue screen -- the final time it took down the whole machine and caused a hard reboot.  X2 (nor any other version of Cakewalk/Sonar) never had any problems so severe as to take down the entire system, particularly in Win7.  
 
The VST3s that are not showing up include Ozone 5 and Celemony, both of which I bought and have been using for a year or more.

 
The reboot of the machine sounds like a BSD

It's not a BSD, which means "Blue Screen of Death."  When the OS catches a serious error, it does a BSD and creates a dump file.  That's not what happened.
 

and would depend on the machine settings if you see the blue screen or the machine will just reboot or as you called it a hard reboot.

I'm sorry, but you are mistaken.  The BSD is an OS feature that permits identifying problems by examining the dump file.  A hard reboot cuts power to the machine and restarts it as if it was just turned on.  A soft reboot doesn't cut power and simply causes a jump to the entry point of the BIOS.
 

Did you install the VST3 plugins at the same time as the VST2 plugs and is the folder you installed them in to scanned by Sonar? 

All my VSTs were installed as I acquired them.  And, yes, the folder is scanned by Sonar.

I was having a similar problem before X3 came out.  I had a brand new machine that kept rebooting for no apparent reason.  I sent it back to Magic Micro (where I had it built) and they replaced the ram, but when they sent it back I was still having the same problem.  I deleted Bitdefender and started using Windows antivirus and firewall software, I also deleted a bunch of non-essential software that "phoned home", like auto update software for Adobe.  After that I haven't been having any kind of problem.

For the plugins in X3, I was pleasantly surprised that it not only found ALL of my previous plugins from earlier Sonar versions, but it arranged them in a very orderly manner unlike before,  the VST plugins are now arranged in groups according to uses (vocals, mixing, guitar amp sims, etc.)  I thought that was a very nice touch!

PTravel
After installation (an upgrade from X2), none of my VST3s showed up, nor did some VST2s.  I tried the "Reset and rescreen" trick recommended by Cakewalk, but that didn't work either.  Worse, Sonar crashed a couple of times in the process, usually when I tried to use the plug-in manager for scanning.  The last time, Sonar not only crashed, but it took down my computer -- everything went black and the machine rebooted.  This is the first time I've seen ANY software mess up so badly that it took Win7 down without a BSD.
 
I'm going to try uninstalling X3, uninstalling X2, doing a manual registry scrub and file delete from User Data and then re-install.
 
I've never had this kind of trouble with a Cakewalk product, and I've been using this DAW since it wasn't yet a DAW and just a sequencer called Cakewalk.

Are you by any chance using any UAD cards?