I've been saving my current project frequently, just made several edits and went to save, and after attempting to save got the error message: Cannot write - disk may be full.  
 
My disk is nowhere near full.  I tried saving it as another file name, and to another drive, and still get the same error message.  I don't want to close Sonar for fear of losing all the edits I just made.  Any ideas?

Found this old post:
 

Chappel
I've seen that error message a few times. A reboot always took care of it. One time, after getting that error message, I wanted to see if I could export an audio track as a wave file and was able to do that with no problems. Then I tried to save the project and it saved normally. The next time I got the "Can't Write File - Disk May Be Full" error message, the first thing I did was export an audio track as a wave file. Again, I was able to save the project normally. I don't get that error message often but now when I do get it I just export an audio track as a wave file and continue to march. I don't know if this will work for others because there may be different causes involved, but it works for me.

 
That actually worked for me - exported audio as a wave file, then it saved fine!!

Defragging the audio drive always seemed to be the solution to this.