I'm not a heavy user and struggle to understand DAWs.  After upgrading from X2 to X3a, (probably a dubious choice given my level of use) several frustrations arose that never showed up in X2.
 
1. Steinberg UR28M all of a sudden needed eLicense after 2 years of X2 operation... now I have the license thanks to Yamaha support, but...  
2. The Audio FX list, while still visible as it was in X2 when I used various reverbs, will allow me to open none of the effects when I click on them; A blue window pops up when SONAR is booted to tell me 93 VST plug-ins were detected.  Right.   I don't know where they are... and...
3. Tonight, after using the Steinberg only three days ago, SONAR cannot even "see" the UR28M in the Devices pane for me to check... a message says there's no audio interface.  The UR28M power light comes on, but other indicator lights do not... and I can't power the UR28M off with the power button as in times past. 
 
This is enough to make me uninstall X3 and go back to X2, but I can't be sure it'd work either. I could at least use the UR28M the other day with X3... why did it stop working?!
 
Btw... In order to use the system, I've plugged in and am using the Scarlett 18i6 without any problems.  Unfortunately, the Audio FX are still not available. 
 
Now I'm trying to research what X3e is vs. X3a.  Will try to find and upgrade I guess.  And I'll continue to search the forum for answers.

I do not know anything about your issue but you definitely should upgrade to X3e because there is alot of bug fixes and for most people it was a very stable release. You should also contact Cakewalk support because they will be able to help.
 
On your system you can also have both X2 and X3 installed and use both while you sort your issues out.

Thank you for the help.  Before I posted this note, a detour back to X2 had already failed to solve any of the issues.  However, the X3e upgrade looks like the thing to do.  If that doesn't work, I may download any updates to X2.  So far it was more stable than X3.  I'll contact Cakewalk support this week.

The UR28m works just fine with X3.
 
I suggest the following steps:
 
Power off UR28m.
 
Uninstall X3, reboot, then install again (Run as Administrator). 
 
Download and install the X3e update (Run as Admin).  Reboot.
 
Power on UR28m.
 
In Sonar, under Preferences - VST settings, change scan to Manual, then
do a Reset and Rescan of VST files. (This is very important).
 
Also under Preferences, verify that the UR28m is showing up with all correct I/O and buffer settings.  
 
Test out Sonar X3e.  Report back with your findings.  Good luck!

+1
 
X2A vs X3E in terms of stability. No comparison... X3E wins every time...
I'd also be looking at updating your drivers and firmware everywhere... And I would run windows update (several times if you haven't done it recently) including optional updates.

All this great advice may turn me into a heavy SONAR user!  Here's the latest:
 
I updated to X3e yesterday before I viewed your posts today.  Rebooted and the Steinberg purred like a kitten.  Have done some tracking with it even before seeing the fine tuning suggestions for the I/O and buffer settings.  FXs have now become accessible, as well.  I'm back in working order and pleased.
 
As for I/O and buffers, at your advice I've changed both playback and record buffers to 128 from the 512 settings I've been using for both for a long time.  I was already set at 24 for audio bit depth, but just now checked the 64-bit double precision engine box (or should I have left it unchecked?)  I nudged latency to 360 from about the 200 position (midway spot) it was set at.
 
I'll check out these settings with some test recordings in the next few days when I can get back in the studio for a while.  Any feedback on the effects you'd predict from the positions these setting were at compared to your suggested settings?
 
As for the Manual VST scans, I could do that, but the Automatic scan function is working well and I'm reluctant to permanently set it on Manual.  What am I missing in doing so?  Might I simply change to Manual, do a Reset and Rescan only if I encounter problems again?
 
You've all been a big help!  Thanks.

ddnoack@q.com
....  Might I simply change to Manual, do a Reset and Rescan only if I encounter problems again?
 ...

Yes, if it ain't broke....
 
As for the other settings, of course you can experiment.  64-bit double precision and lower buffers will push your system harder, so tweak those first if needed.  
 
Otherwise, sounds like you're on the right track.  Welcome to the forum!

Thanks... While you're on the line... Noticed a new problem last night in playing a project I'd checked out before the above issues, moving the channel faders down below 0db to trim things up was interrupted a dozen or so times by the fader automatically jumping up to 0db.  I'd move them back down and 10 seconds (or some random number of seconds) later, the fader would magically reset to 0db...it's favored position apparently.  Ideas?  I didn't change anything, but obviously the update to X3e changed a setting. 
 
I'll investigate further on my own, but thought you might have a suggestion. 

ddnoack@q.com
Noticed a new problem last night in playing a project I'd checked out before the above issues, moving the channel faders down below 0db to trim things up was interrupted a dozen or so times by the fader automatically jumping up to 0db.  I'd move them back down and 10 seconds (or some random number of seconds) later, the fader would magically reset to 0db...it's favored position apparently.  Ideas?  I didn't change anything, but obviously the update to X3e changed a setting. 
 
I'll investigate further on my own, but thought you might have a suggestion. 

That really sounds like a volume automation envelope exists, which can override the fader position.  Expand the automation lanes on that track and see whether a volume envelope exists and has nodes at different levels. 
 

ddnoack@q.com
Might I simply change to Manual, do a Reset and Rescan only if I encounter problems again?
 

In fact it is recommended, full info here:
http://www.cakewalk.com/Support/Knowledge-Base/2007013315/SONAR-X3-Full-VST-Rescan-Recommended-for-Upgraders
 
Cheers...