The Save (Not Permitted) dialog box warns you that saving changes is not permitted because the changes you have made require the listed tables to be dropped and re-created.
The following actions might require a table to be re-created:
Adding a new column to the middle of the table
Dropping a column
Changing column nullability
Changing the order of the columns
Changing the data type of a column
To change this option, on the Tools menu, click Options, expand Designers, and then click Table and Database Designers. Select or clear the Prevent saving changes that require the table to be re-created check box.
http://msdn.microsoft.com/en-us/library/bb895146.aspx
