stevec
X3 32bit or 64bit? And in either case, where are your VST path(s)? You could reinstall 100 times, but if the VSTs are not located where SONAR is pointing they'll never be found. The fact that you lost one synth by simply installing a different one doesn't make sense, unless there's a pathing issue adding to the situation.
My X3 is 64 bit as was my last working installation of X2a.
The only difference between these addresses that I can see is capitalisation.
The VST settings path is:
c:\program files\cakewalk\vstplugins\session drummer 3
The file SessionDrummer x64.dll on my computer is at:
C:\Program Files\Cakewalk\Vstplugins\Session Drummer 3
The address for session drummer does not appear in the list of enabled plugins in the plugin manager.
A quick check with the most recent x2a 64 bit reveals that it too in minus Session Drummer.