For quick response of NCrunch, I have set its workspace to a RAM disk. However, its size is limited, so it can happen that the disk is full (ex. if I have multiple large solutions open). In that case Ncrunch displays an "unknown build error" and in the details it states that the disk is full.
Perhaps you could catch that error and try to clean up (if possible). Also you could display that error in a more user-friendly way (ex. popup "disk full" or something like that).