I don't have the solution, but I think I have the cause.
Are you working with a laptop or computer that connects and disconnects to a company domain
And this occurs when you are trying to create a Database Diagram while following along with "Absolute Beginner’s Video Series"
I bet you downloaded C# Express or VB Express and the video series at work - but like any good employee - you went home to watch the videos and start programming.
I think the problem is the dbo (data base owner) user name gets stored with the database when you create it in C# Express. Or maybe the dbo user is set when you first install C# or VB Express Edition.
I installed both VB and C# Express right after I downloaded both while at work - onto my laptop - while connected to the company network. So my dbo user name is "SOME_COMPANY_DOMAINNAME/user_name". But when I run C# or VB at home - I log in as to Windows as plain "user_name". So at home I can not get the Database Diagrams function to work because I am now "user_name" and it's looking for "SOME_COMPANY_DOMAINNAME/user_name"!
In C# I right click on the database in the Database Explorer and select "Modify Connection". I see there that "Use Windows Authentication" is selected. And when I click on the database (in this case it's rss.mdf since I'm following the video series to create the RSSReader application), and look at the properties, near the bottom it shows the "Owner" as "SOME_COMPANY_DOMAINNAME/user_name".
The strange thing is when I'm at home, and not connected to the company network, and logged in as "user_name", I can still establish a connection to the rss.mdf database - I just can't create a Database Diagram!!
So somebody tell us how we can change the dbo user name or allow us to create the Database Diagram that is asked for as part of "Absolute Beginner’s Video Series" ! Because we are "absolute beginners" and we would like to become proficient C# programmers someday.