In Dreamweaver CC, related files, shown as tabs at the top of the currently open file window, now fail to open their file content. So clicking on e.g. header.php would have brought up the content for header.php, now a message appears saying "header.php is not on the local disk". Opening it in the file list shows the file contents perfectly. This only started to fail today. The same file still opens from the tab correctly in CS6 so something in CC is causing the problem. All site files are on an external drive permanently connected to a Mac BookPro, and they have been for years with no issues. Any ideas why this problem started?