Diagram opens read-only
A diagram can open as read-only for the following reasons:
• The Open Diagrams Read-only check box is selected for the Diagrams entry on the Tool Options dialog. If this is the case, you can release the lock and change the diagram through the 
Lock Diagram for Edit button on the Format tab.
Lock Diagram for Edit button on the Format tab.• Another user has the diagram open, in which case a message informs you which user has the diagram locked. You can ask the user that has the diagram locked to close the diagram or release the lock.
• The diagram is mistakenly locked. This can happen if a user has the diagram open and their computer crashes.
If the diagram is mistakenly locked, you can use Model Explorer to remove all locks from the diagram's Model.