Having just moved my project into Revit 2010 I have now twice had this error...
Linked file ..\..\..\filename.dwg could not be located. It will be displayed as it was when this project was last saved.
Only it does not display the file. And what's more Revit will not let you locate it and reload it. The only way out is to unload the file and reload it.
This has happened when the DWG has been edited, and then the Revit file re-opened or the link re-loaded.
Any one had similar problems or knows what is going on?