--- lisp/files.el	8 Aug 2007 14:06:01 -0000	1.896.2.15
+++ lisp/files.el	22 Aug 2007 03:47:35 -0000	1.896.2.16
@@ -3120,7 +3120,12 @@
 	    (file-error nil))))))
 
 (defun backup-buffer-copy (from-name to-name modes)
-  (let ((umask (default-file-modes)))
+  (let ((umask (default-file-modes))
+	(dir (or (file-name-directory to-name)
+		 default-directory)))
+    ;; Can't delete or create files in a read-only directory.
+    (unless (file-writable-p dir)
+      (signal 'file-error (list "Directory is not writable" dir)))
     (unwind-protect
 	(progn
 	  ;; Create temp files with strict access rights.  It's easy to