This blog describes another practical scenario where you are not able to edit the sharepoint page in Advanced mode from SharePoint Designer 2010.
Issue :
Not able to Edit the Advance Mode From SharePoint Designer 2010
Resolution :
1.http://site-name/_layouts/15/SharePointDesignerSettings.aspx is the page you need to work on.
2.To access the page , Go to Central Administration.
a. Central Administration - Web Applications- Manage Web Applications - Select the Web Application.
b. From Ribbon Navigate to Manage >General Settings > SharePoint Designer.
3. In the SharePoint Designer section, Check the "Enable Detaching Pages from the Site Definition".
That's it folks. The Page can be easily edited now.
Happy SharePointing :-)