Access denied. You do not have permission to access this content

You have created an External Content Type in SharePoint Designer 2010 with the authentication mode "User's Identity". When you create a list and want to access the data in your portal, you get the error message "Access denied. You do not have permission to access this content" or "Access denied by Business Data Connectivity".

You need to set permissions in Central Administration under "Manage Service Applications" in the "Business Data Connectivity Service Application". Click the BCS service and configure the permissions in the context menu of your ETC.

Also the user needs permissions in your external database.