diff --git a/drivers/md/md.c b/drivers/md/md.c
index e2782a04012d..00788c56276f 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1891,20 +1891,44 @@ static ssize_t
 slot_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 {
 	char *e;
+	int err;
+	char nm[20];
 	int slot = simple_strtoul(buf, &e, 10);
 	if (strncmp(buf, "none", 4)==0)
 		slot = -1;
 	else if (e==buf || (*e && *e!= '\n'))
 		return -EINVAL;
-	if (rdev->mddev->pers)
-		/* Cannot set slot in active array (yet) */
-		return -EBUSY;
-	if (slot >= rdev->mddev->raid_disks)
-		return -ENOSPC;
-	rdev->raid_disk = slot;
-	/* assume it is working */
-	rdev->flags = 0;
-	set_bit(In_sync, &rdev->flags);
+	if (rdev->mddev->pers) {
+		/* Setting 'slot' on an active array requires also
+		 * updating the 'rd%d' link, and communicating
+		 * with the personality with ->hot_*_disk.
+		 * For now we only support removing
+		 * failed/spare devices.  This normally happens automatically,
+		 * but not when the metadata is externally managed.
+		 */
+		if (slot != -1)
+			return -EBUSY;
+		if (rdev->raid_disk == -1)
+			return -EEXIST;
+		/* personality does all needed checks */
+		if (rdev->mddev->pers->hot_add_disk == NULL)
+			return -EINVAL;
+		err = rdev->mddev->pers->
+			hot_remove_disk(rdev->mddev, rdev->raid_disk);
+		if (err)
+			return err;
+		sprintf(nm, "rd%d", rdev->raid_disk);
+		sysfs_remove_link(&rdev->mddev->kobj, nm);
+		set_bit(MD_RECOVERY_NEEDED, &rdev->mddev->recovery);
+		md_wakeup_thread(rdev->mddev->thread);
+	} else {
+		if (slot >= rdev->mddev->raid_disks)
+			return -ENOSPC;
+		rdev->raid_disk = slot;
+		/* assume it is working */
+		rdev->flags = 0;
+		set_bit(In_sync, &rdev->flags);
+	}
 	return len;
 }
 
@@ -5549,6 +5573,7 @@ static int remove_and_add_spares(mddev_t *mddev)
 
 	ITERATE_RDEV(mddev,rdev,rtmp)
 		if (rdev->raid_disk >= 0 &&
+		    !mddev->external &&
 		    (test_bit(Faulty, &rdev->flags) ||
 		     ! test_bit(In_sync, &rdev->flags)) &&
 		    atomic_read(&rdev->nr_pending)==0) {