@@ -146,46 +146,53 @@ func getBuiltinDriver(name, home string, options []string, uidMaps, gidMaps []id

 }

 // New creates the driver and initializes it at the specified root.

-func New(root string, name string, options []string, uidMaps, gidMaps []idtools.IDMap) (driver Driver, err error) {

+func New(root string, name string, options []string, uidMaps, gidMaps []idtools.IDMap) (Driver, error) {

 	if name != "" {

 		logrus.Debugf("[graphdriver] trying provided driver %q", name) // so the logs show specified driver

 		return GetDriver(name, root, options, uidMaps, gidMaps)

 	}

 	// Guess for prior driver

-	priorDrivers := scanPriorDrivers(root)

+	driversMap := scanPriorDrivers(root)

 	for _, name := range priority {

 		if name == "vfs" {

 			// don't use vfs even if there is state present.

 			continue

 		}

-		for _, prior := range priorDrivers {

+		if _, prior := driversMap[name]; prior {

 			// of the state found from prior drivers, check in order of our priority

 			// which we would prefer

-			if prior == name {

-				driver, err = getBuiltinDriver(name, root, options, uidMaps, gidMaps)

-				if err != nil {

-					// unlike below, we will return error here, because there is prior

-					// state, and now it is no longer supported/prereq/compatible, so

-					// something changed and needs attention. Otherwise the daemon's

-					// images would just "disappear".

-					logrus.Errorf("[graphdriver] prior storage driver %q failed: %s", name, err)

-					return nil, err

-				}

-				if err := checkPriorDriver(name, root); err != nil {

-					return nil, err

+			driver, err := getBuiltinDriver(name, root, options, uidMaps, gidMaps)

+			if err != nil {

+				// unlike below, we will return error here, because there is prior

+				// state, and now it is no longer supported/prereq/compatible, so

+				// something changed and needs attention. Otherwise the daemon's

+				// images would just "disappear".

+				logrus.Errorf("[graphdriver] prior storage driver %q failed: %s", name, err)

+				return nil, err

+			}

+

+			// abort starting when there are other prior configured drivers

+			// to ensure the user explicitly selects the driver to load

+			if len(driversMap)-1 > 0 {

+				var driversSlice []string

+				for name := range driversMap {

+					driversSlice = append(driversSlice, name)

 				}

-				logrus.Infof("[graphdriver] using prior storage driver %q", name)

-				return driver, nil

+

+				return nil, fmt.Errorf("%q contains several valid graphdrivers: %s; Please cleanup or explicitly choose storage driver (-s <DRIVER>)", root, strings.Join(driversSlice, ", "))

 			}

+

+			logrus.Infof("[graphdriver] using prior storage driver %q", name)

+			return driver, nil

 		}

 	}

 	// Check for priority drivers first

 	for _, name := range priority {

-		driver, err = getBuiltinDriver(name, root, options, uidMaps, gidMaps)

+		driver, err := getBuiltinDriver(name, root, options, uidMaps, gidMaps)

 		if err != nil {

-			if err == ErrNotSupported || err == ErrPrerequisites || err == ErrIncompatibleFS {

+			if isDriverNotSupported(err) {

 				continue

 			}

 			return nil, err

@@ -194,9 +201,10 @@ func New(root string, name string, options []string, uidMaps, gidMaps []idtools.

 	}

 	// Check all registered drivers if no priority driver is found

-	for _, initFunc := range drivers {

-		if driver, err = initFunc(root, options, uidMaps, gidMaps); err != nil {

-			if err == ErrNotSupported || err == ErrPrerequisites || err == ErrIncompatibleFS {

+	for name, initFunc := range drivers {

+		driver, err := initFunc(filepath.Join(root, name), options, uidMaps, gidMaps)

+		if err != nil {

+			if isDriverNotSupported(err) {

 				continue

 			}

 			return nil, err

@@ -206,31 +214,21 @@ func New(root string, name string, options []string, uidMaps, gidMaps []idtools.

 	return nil, fmt.Errorf("No supported storage backend found")

 }

+// isDriverNotSupported returns true if the error initializing

+// the graph driver is a non-supported error.

+func isDriverNotSupported(err error) bool {

+	return err == ErrNotSupported || err == ErrPrerequisites || err == ErrIncompatibleFS

+}

+

 // scanPriorDrivers returns an un-ordered scan of directories of prior storage drivers

-func scanPriorDrivers(root string) []string {

-	priorDrivers := []string{}

+func scanPriorDrivers(root string) map[string]bool {

+	driversMap := make(map[string]bool)

+

 	for driver := range drivers {

 		p := filepath.Join(root, driver)

 		if _, err := os.Stat(p); err == nil && driver != "vfs" {

-			priorDrivers = append(priorDrivers, driver)

+			driversMap[driver] = true

 		}

 	}

-	return priorDrivers

-}

-

-func checkPriorDriver(name, root string) error {

-	priorDrivers := []string{}

-	for _, prior := range scanPriorDrivers(root) {

-		if prior != name && prior != "vfs" {

-			if _, err := os.Stat(filepath.Join(root, prior)); err == nil {

-				priorDrivers = append(priorDrivers, prior)

-			}

-		}

-	}

-

-	if len(priorDrivers) > 0 {

-

-		return fmt.Errorf("%q contains other graphdrivers: %s; Please cleanup or explicitly choose storage driver (-s <DRIVER>)", root, strings.Join(priorDrivers, ","))

-	}

-	return nil

+	return driversMap

 }