Here's everything I've gathered so far. Method 10 definitely works but it's a dog!
 
1) Export any audio track as a wave file, then try to Save.
2) Use Save As.
3) Double disk I/O buffers in preferences.
4) Try saving the file to the root of the drive in question in case deep nesting is the problem
5) Delete the picture cache
6) Double "DiskRecBufSize" in Aud.ini (to 512)
7) Consolidate Audio under Tools.
Then go to the Audio folder and delete everything EXCEPT the backup folder.
Open the backup folder and move the saved audio to the Audio folder.
Delete the empty backup folder.

8) Run CheckDisk on SONAR drive ('Disk may be full' is probably not a disk issue though)
9) Reboot
10) Copy all contents to a new file.
For some reason this is not the same as Save As. It's worked for me in the past but it's a long and tedious process
if you have a lot of tracks.
a) Open a new file and save it with a new name. Open the problem file.
b) Drag the source file aside so you can see both.
c) Recreate the source tracks in the destination file and label them all just as they are in the source
d) Now drag copy (Cntrl/drag) all the effects plugs one by one from source file tracks to the corresponding destination tracks
e) Set the volume and pan settings in the destination tracks so they're the same as the source
f) Copy (Cntrl/drag) the audio and MIDI data one track at a time to the prepared destination tracks
g) Add the same synths to the Synth Rack destination file and copy any settings such as drum kit presets, fader settings, articulations, etc
Keep saving the destination file as you work. When you've finished you should have two identical files with identical plugs, settings and synths. I don't know why this works or how it's different to Save As but the new file won't give you the 'Cannot write' error.
The only reason I'm looking for another solution is because method 10 is so long and tedious.

The drive buffer setting is the same as the DiskRecBufSize you have listed.

I think it tells Sonar how much audio it buffers en routento writing it to the drive.

Another related setting is whether or not to let Sonar over-ride Windows' normal write routine )where it waits for a suitable time slot to save a file) and instead send the daya to the drive as it records. The latter being what Sonar, like most DAWs, does by default, the idea being that even if Sonar or Windows crashes mid session the recording will be on the drive come what may - which isn't always successful.

My "gut feeling" is that this issue may well have more than one cause and, as you say, Sonar then comes up with the most relevant error message it has but the message doesn't explain what's really happened. A bit like that MS favourite "an error has occured" which is helpfully flashed up but contains no information about what the error is. It seems to plague a few people but most, including myself, never actually see it on our own setups.

Is it possible you resurrected that file/folder from an archive? Some archiving functions will set the contents to "read only" and I am now wondering if it may be something similar. Have you tried changing the properties on that project folder by unchecking "read only" (project cannot be open to set it)?
 
If you can create and save a new project without issues, I am struggling to believe this is within SONAR.