The Set Up: Currently on my contract, we have several farms, one of which is an internal portal that is used for employee only collaboration. It requires an Active Directory account and the layout of the environment consists of a main Site Collection for each bureau and sub sites created for each different department below that as they are requested.
The Problem: A site owner contacted me unable to edit a site page using SharePoint Designer. he sent me the following screenshots (names have been blurred to protect the innocent):
What this error tells us is that the user does not have permissions to the custom master page that we are using on the site collection. Since it is a sub site that inherits it's master page from the top level site collection, we are having a permissions issue.
The Details: As you will read in this Microsoft support article: http://support.microsoft.com/kb/2592376
Since I cannot give this site owner site collection admin, owner or designer permissions to my top-level master page library, and I was unable to get the site to recognize the sub site's master page library in the master page settings, the user was getting the error above. Simply creating a new site collection for the user and migrating the data was too much effort, so I had to be a bit tricky with how to make this work.
The Solution: I created a new folder in the top-level master page library and broke inheritance to the folder, giving him designer permissions. I took a copy of the master page and placed it in that folder, then set the site (and all sub-sites) to inherit this new master page. He now had the necessary permissions to open site page and edit the content without the master page error, and I was able to maintain security by not allowing him elevated access to the real master page library.
I'm not sure if this is the best way, but this fix took all of 5 minutes to do and the user had all the functionality he needed and was back in business without any impact to the end-user look & feel (or needing to migrate content from one site to a new site collection). If anyone has any other suggestions or perhaps a better way of being able to get the sub site to recognize it's own master page library (using the "_catalogs/masterpage/Forms/AllItems.aspx" string), please feel free to post a comment in the section below.