I have my own patterns in the PRV but when I hit play, Sonar is playing by my patterns AND SD's demo patterns.
How can I turn this internal sequencer off so that Session Drummer only plays my patterns?

I'm not at my daw right now but I can tell you that absolutely freaked me out when it happened to me!

I agree that it's quite annoying. It's the #1 reason I stopped using it and moved on to another drum program. (Which is a shame because the sound packs for it are pretty good)

Not sure if this is what is happening, but the 8 internal patterns (A->H) can be triggered by hitting MIDI notes (and I forget which ones offhand). It is possible that you have those MIDI notes in your SS pattern.
 
Only "sure-fire" way I can think of is to right click each of the SD3 patterns (A->H) and select "Unload Pattern." Then SD3 has nothing to fire on you. Afterwards, you may want to also save that bank preset with a new name so SD3 doesn't reload those patterns on you later.

OK, found the problem.
If I click the Play button on Session Drummer and let it play through a pattern, the Play button is "remembered".
If I then go to Sonar and press the Play button on the top panel to start the song, SD's demo pattern also plays.
 
But, if, after SD has played through, I hit the Stop button (despite the fact that SD has actually stopped on its own), then the Stop button is "remembered" and it's demo sequencer will NOT start.
 
Very confusing especially when it has worked fine before.

Thanks for the assistance, everyone.