This error message can also happen with Word on the Mac, so I don't think it's solely a Windows issue. I've also had it happen a few times with Windows when saving to a USB drive. For some reason no one has been able to explain to me, sometimes a drive will become 100% read-only for no apparent reason. This makes it impossible to save to, copy, or modify the drive's contents. There is a solution to this -
bookmark this article.
That said, I agree that all programs, most definitely including Sonar, should
never fail to save a file properly unless it's a hardware issue. However in my experience of working with computers since the late 70s, I've had corrupted file saves several times, with several different programs. As a result I do not trust ANY software to save 100% of the time without errors.
Remember that backing up is different from archiving. To back up with Sonar (assuming you save all files to a per-project folder), just copy the entire folder to a different drive or even a USB stick whenever you've reached a point where you don't want to lose something. Archiving to DVD or whatever can be done after the session.