Index was outside the bounds of the Array. (Microsoft.SqlServer.smo)
Solution 1:
Restarting the Management Studio worked for me.
Solution 2:
Upgrade your SqlServer management studio from 2008 to 2012
Or Download the service packs of SqlServer Management Studio and update probably resolve you solution
You can download the SQL Server Management studio 2012 from below link
Microsoft® SQL Server® 2012 Express http://www.microsoft.com/en-us/download/details.aspx?id=29062
Solution 3:
For me this problem still exists with SSMS version 2016 (13.0.16100.1).
A decent workaround is to not use the 'Right click' -> 'Add table...' dialog, but simply drag the table you want to add from the Object Explorer, onto the Diagram surface. While dragging the mouse icon changes into an 'add' symbol and the table is added when you release the mouse.
Beats having to close SSMS every time.
Solution 4:
Restarted worked! I found the same error to add new table to my database diagram on sql server 2016, restarted sql server management studio, finally solved.
Solution 5:
This is an issue if you are using 2008 management studio tools to connect to a SQL 2012 instance.
I experience this a lot if I am working on one server with SQL 2008, and trying to quickly query another server that is running SQL 2012.
I normally keep my personal workstation on the latest version of management studio (2012 in this case), and am able to administer all servers from there.