Occasionally, when saving edited diagrams in Management Studio, the save takes from a few minutes to 20+ minutes. The changes to the tables are saved right away but the diagram is not saved and clicking on the diagram results in a popup at the notification area saying that Management Studio is waiting ofr an internal operation to complete.
Opening another instance of Management Studio and looking at the database shows that the table changes are saved but the diagram has not been saved. When the "internal operation" eventually finishes, the diagram changes are reflected in the database.
In the current new database I am developing, this has occurred twice (so far). This is a new database and has no data. I have been working on the database for a week.
In both occurences, among the changes made, I had created a bit column and assigned that bit column a 0 default value in the property page. I don't know if that is related or not but nothing else stands out as being unique in the changes being saved. These are, so far, the only two bit columns in the database.
During the delay, I run sp_who and sp_who2. The results of that shows that every connection to my target database is in sleeping state. That tells me that the problem is not a database locking or timeout issue but that the problem is, as the notification states, an internal operation in Management Studio.
The other changes being saved, both times that I had the error and many, many times that I did not, generally included new columns in existing tables, adding new tables, and adding new relationships as well as whatever moving objects around on the diagram may have taken place.