Any clues?
Illustrator 10 .ai files are based on PDF format (unlike version 8 and prior which were based on EPS). How long is the character count of the path from drive to .ai"? Do you have any extended characters anywhere in that path, including "foreign" accented letter characters?
Bob
If you are having problems with corrupt files or missing files or Save not working from Illustrator 9/10 there may be a solution. It turns out that if your .AI file is in a path with non-English characters or if the path is too long you will get a strange error about having problems with writing the .PDF file format from Illustrator 9/10. See the Adobe document 325878 as found at <http://www.adobe.com/support/techdocs/26b3e.htm> for details. Other save problems can come from moving files and folders around from their original locations.
no matter what i do. change the name, change the location, etc.
If you have "tried all the fixes" including all those suggested above, the I recommend you call Adobe Technical Support by phone and ask to talk to a second level support person. This is not Adobe but other Users just like you.