Changes after the feedback from @catsby - these all made perfect sense

builtin/providers/aws/resource_aws_rds_cluster.go

@@ -127,12 +127,17 @@ func resourceAwsRDSCluster() *schema.Resource {
 			"preferred_backup_window": &schema.Schema{
 				Type:     schema.TypeString,
 				Optional: true,
+				Computed: true,
 			},
 
 			"preferred_maintenance_window": &schema.Schema{
 				Type:     schema.TypeString,
 				Optional: true,
+				Computed: true,
 				StateFunc: func(val interface{}) string {
+					if val == nil {
+						return ""
+					}
 					return strings.ToLower(val.(string))
 				},
 			},

builtin/providers/aws/resource_aws_rds_cluster_test.go

@@ -20,6 +20,27 @@ func TestAccAWSRDSCluster_basic(t *testing.T) {
 	ri := rand.New(rand.NewSource(time.Now().UnixNano())).Int()
 	config := fmt.Sprintf(testAccAWSClusterConfig, ri)
 
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSClusterDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: config,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSClusterExists("aws_rds_cluster.default", &v),
+				),
+			},
+		},
+	})
+}
+
+func TestAccAWSRDSCluster_backups(t *testing.T) {
+	var v rds.DBCluster
+
+	ri := rand.New(rand.NewSource(time.Now().UnixNano())).Int()
+	config := fmt.Sprintf(testAccAWSClusterConfig_backups, ri)
+
 	resource.Test(t, resource.TestCase{
 		PreCheck:     func() { testAccPreCheck(t) },
 		Providers:    testAccProviders,
@@ -41,12 +62,12 @@ func TestAccAWSRDSCluster_basic(t *testing.T) {
 	})
 }
 
-func TestAccAWSRDSCluster_update(t *testing.T) {
+func TestAccAWSRDSCluster_backupsUpdate(t *testing.T) {
 	var v rds.DBCluster
 
 	ri := rand.New(rand.NewSource(time.Now().UnixNano())).Int()
-	preConfig := fmt.Sprintf(testAccAWSClusterConfig, ri)
-	postConfig := fmt.Sprintf(testAccAWSClusterConfig_update, ri)
+	preConfig := fmt.Sprintf(testAccAWSClusterConfig_backups, ri)
+	postConfig := fmt.Sprintf(testAccAWSClusterConfig_backupsUpdate, ri)
 
 	resource.Test(t, resource.TestCase{
 		PreCheck:     func() { testAccPreCheck(t) },
@@ -147,8 +168,16 @@ func testAccCheckAWSClusterExists(n string, v *rds.DBCluster) resource.TestCheck
 	}
 }
 
-// Add some random to the name, to avoid collision
 var testAccAWSClusterConfig = `
+resource "aws_rds_cluster" "default" {
+  cluster_identifier = "tf-aurora-cluster-%d"
+  availability_zones = ["us-west-2a","us-west-2b","us-west-2c"]
+  database_name = "mydb"
+  master_username = "foo"
+  master_password = "mustbeeightcharaters"
+}`
+
+var testAccAWSClusterConfig_backups = `
 resource "aws_rds_cluster" "default" {
   cluster_identifier = "tf-aurora-cluster-%d"
   availability_zones = ["us-west-2a","us-west-2b","us-west-2c"]
@@ -160,8 +189,7 @@ resource "aws_rds_cluster" "default" {
   preferred_maintenance_window = "tue:04:00-tue:04:30"
 }`
 
-// Add some random to the name, to avoid collision
-var testAccAWSClusterConfig_update = `
+var testAccAWSClusterConfig_backupsUpdate = `
 resource "aws_rds_cluster" "default" {
   cluster_identifier = "tf-aurora-cluster-%d"
   availability_zones = ["us-west-2a","us-west-2b","us-west-2c"]