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.

Thanks Chappel, that worked for me. First time I've had this message

error came up and the audio export workaround seems to have done the trick.  Sonar would not even save the project to a completely new hard drive.  It would copy all the audio files no problem but balk at saving the project file.  Good thing there's a workaround...