Some of my older projects (done in X3) will not open in Platinum. I have a different interface now than I did then (Tascam 20X20). I am able to open some of them on my laptop with no interface hooked up and it gripes about silent busses etc. but still opens. Is there a work around other than importing the files to another project? I don't want to lose all my mix work and detail settings. Just wanted to try some things before I have to rebuild all

Sometimes this is due to older plugins. Open in Safe Mode and then go through the tedious task of deleting incompatible Vst's (plugins) , seeing that you don't want to totally remix your projects.
You may have to replace some plugins, but hopefully you won't have to remix entire project.

Sounds like all you need to do is re-route the old projects to the new interface. The silent-bus messages are most likely simply because the project references a now-non-existent output devices. I went through the process a year ago, updating many of my old projects after my interface died and had to be replaced. 

Yes the silent bus message is par for the ccourse when you switch interfaces. But you should be able to open them. The only other reason is if the sample rate has changed but that too would just be a warning. 
Try safe mode. 
What was the old interface? Did it use a radically different in/ out? 
I switch interfaces all the time and also work without one from time to time when travelling.
ALl I get is the Silent buss warning. 
Open Sonar without a project and switch to WDM mode, that's what I do when I don't have my interface. 

Thanks.  Some good things to try. My old interface was actually really close to the same thing. Tascam US1641. How do you open in "safe mode"?

Hold down the shift key whilst opening a project

re:  the silent busses.  The shift key may work when opening, but it sounds like there may be some routing problem that the new Tascam can't do w/o help.  If you can open the project just poke around the SONAR Mixer.

The 2 interfaces I switch between without issue are the TAscam us1641 and my Scarlett 6i6. 
Funny your having this problem.. 
Did you try WDM mode? 

Have not tried WDM mode yet. I did however open a whole slew of projects from that same era that had nothing but WAVs in them. It was almost instant it was so fast and there are 20 to 30 tracks per song. I am wondering about the possibility of having 32 bit plugs in those older projects. Still seems strange that SPLAT would just "hang" and not open anyway so I could fix that. BTW I got 1 to open in safe mode and couldn't see anything that was different than recent projects.
 
hmmmm......
 

http://forum.cakewalk.com/Splat-not-responding-m3352542.aspx
 
 
WOW!   This fixed my problem. When I plugged my Faderport back in it acted like a messenger. When the project got stuck trying to open I just moved the slider a couple of times and Sonar spawned a window every time telling me what was wrong. Crazy but solved