If you try to open the Forms Authentication enabled site in SharePoint Designer it will through the following error:
The folder ‘http://myserver.com/’ isn’t accessible. The folder may be located in an unavailable location, protected with a password, or the filename contains a / aut .
Designer cannot open the site because the site using custom authentication.
There is two workaround for this issue.
Accede 1:
· While login to forms authentication site check the sign in me automatically(it will cache the credential and it will load the Designer)
Accede 2:
· Extend the Existing Forms Authentication Enabled web application to windows authentication application. Now windows authentication enabled web application can be opened in sharepoint designer using windows authentication.
My Reccomandation would be Approach 2
Aliquam pharetra