revnice1
I searched the forum for other posts because this used to be a common problem but I only found my own post.

Try a Google search like "site:forum.cakewalk.com save disk full" and you should get more results
 

Thanks! I need more results!

I recently had the same message and in my case it had nothing to do with disk space. I had been moving files from a memory stick and at one point I got the message in the OP. Sonar was trying to access the memory stick that was no longer in my USB port. Perhaps in this case it's something similar.

That is an interesting point. Does "Save as..." work?

I never got that message specifically, but had a "general error" preventing saving that was resolved by doubling my disk I/O buffers in preferences.

I get it sometimes but it always goes away when I try the action again or use save as.  I think it could be the disk is busy at that instant or something affecting the IO of sonar buts its never been an issue that actually prevented me from continuing 
 

I haven't found that SaveAs works, I think you're just saving the problem with a new name. I just read one where the guy got rid of the message by exporting any audio track as a .wav. Once he did that the problem vanished and consistently so. I'm about to try it.

Some times ago I remember this problem being related to the Master Boot Record; MBR, of the disk in question.
Like Long File Names or the file is nested into too many folders. Also a partition drive. 
A corrupted MBR or a virus. Try saving the file to the root of the drive in question.

I'll try that next and I've been keeping notes on every proposed solution but I can't believe Cakewalk never addressed this anywhere. I'm on Sonar 8.5.3 so maybe it's been fixed since then.

It's a long time ago, but I seem to recall Sonar of that vintage, or maybe Windows or an interaction of the two having an issue with importing tagged mp3 files that could result in that error. And it turned up as an error once in a while years ago, but other than the mp3 thing I don't think anyone got to the bottom of it. So it's quite possible there's no single cause.

One suggested solution was to delete the Sonar picture cache and let it build a new one.
Another possibility is that the drive isn't reacting quickly enough which might be the case if the audio spooling drive is allowed to go to sleep while Sonar is running and Sonar announces that error if Windiws doesn't rell it the drive is ready within the time frame Sonar expects.
Or that the drive buffers (not the audio buffer) need increasing in preferences.

>importing tagged mp3
I think Sonar imports mp3's as .wav's.    
 
>delete the Sonar picture cache and let it build a new one.
I can see how there might be something to that because it would alter data in the cwp file, possibly such that it allows a write.
 
>drive is allowed to go to sleep while Sonar is running 
This makes sense too because 'Disk may be full' could be the only response Sonar has when it's unable to access the disk.
 
>Or that the drive buffers
Not so sure about this one because I can't see the reasoning. Where/how would you change drive buffers and what else would be affected?