I was sent a group of dwg files that seem to have been made with Civil 3D. The problem is that almost half of them display the "Drawing file is not valid" message when I try to open them with AutoCAD 2011LT and 2012 Full. The same happens when I try to open them with TrueView 2013. If I try to convert them, TrueView says it there was some data problem opening them.
When opening them in Notepad I see that it says AC1021, so it's saved as a 2007-2009 version. I got no .BAK files, I can't recover the .DWG's and I can't contact those who originally made the files.
I have attached one of the corrupt files for reference.