Do a Save as and switch to a new location temporarily, such as an internal drive. This "File Unknown" error tends to happen when you change a file's name or move it to another location while it's still open, but other causes have been known.
If that doesn't work, copy every single thing in the project file to a brand-new project file, then save that. If that doesn't work either, check -- an assistant editor I trained has reported that on his work stations, the Multiclip function has been known to prevent him from saving the project file.
And make sure you perform manual backups at least once every four hours to guard against this kind of phenomenon.