We had a user who had Design permissions on a subsite trying to edit a page in SharePoint Designer 2010. The user could connect to the site in SPD.
When the user tried to edit a page he received the following error: "You do not have permission to do this operation. Ask your web site administrator to change your permissions and then try again, or log on with a user account that has this permission."
After clicking OK, the user received this Master Page error message: "The Master Page file 'location of master page' cannot be loaded. Attach a different Master Page, or correct the problem in Code view."
Since the Master Page was located at the site collection level and the user was editing in a subsite I checked the user's permissions. Even though the user had Design permissions to the subsite, the user only had Read permissions at the site collection level. So my suspicion was that the user did not have the proper permissions at the site collection level. Thanks to Sue Hernandez's blog post for confirming this and also posting a link to the specific Microsoft KB article to support and confirm. Microsoft explains:
Overall, an user needs to be a member of one of the following groups at the site collection level to be able to use SharePoint Designer and modify SharePoint content:
· Site Collection Administrators