diff --git a/Manage.c b/Manage.c
index 1b2b75a..2d8c916 100644
--- a/Manage.c
+++ b/Manage.c
@@ -762,11 +762,24 @@ int Manage_subdevs(char *devname, int fd,
 						remove_partitions(tfd);
 						close(tfd);
 						tfd = -1;
-						if (update) {
+						if (update || dv->writemostly > 0) {
 							int rv = -1;
 							tfd = dev_open(dv->devname, O_RDWR);
+							if (tfd < 0) {
+								fprintf(stderr, Name ": failed to open %s for"
+									" superblock update during re-add\n", dv->devname);
+								return 1;
+							}
 
-							if (tfd >= 0)
+							if (dv->writemostly == 1)
+								rv = st->ss->update_super(
+									st, NULL, "writemostly",
+									devname, verbose, 0, NULL);
+							if (dv->writemostly == 2)
+								rv = st->ss->update_super(
+									st, NULL, "readwrite",
+									devname, verbose, 0, NULL);
+							if (update)
 								rv = st->ss->update_super(
 									st, NULL, update,
 									devname, verbose, 0, NULL);
diff --git a/mdadm.h b/mdadm.h
index d616966..6a36f43 100644
--- a/mdadm.h
+++ b/mdadm.h
@@ -646,6 +646,8 @@ extern struct superswitch {
 	 *   linear-grow-new - add a new device to a linear array, but don't
 	 *                   change the size: so superblock still matches
 	 *   linear-grow-update - now change the size of the array.
+	 *   writemostly - set the WriteMostly1 bit in the superblock devflags
+	 *   readwrite - clear the WriteMostly1 bit in the superblock devflags
 	 */
 	int (*update_super)(struct supertype *st, struct mdinfo *info,
 			    char *update,
diff --git a/super0.c b/super0.c
index 62c4ff0..f791e9d 100644
--- a/super0.c
+++ b/super0.c
@@ -570,6 +570,10 @@ static int update_super0(struct supertype *st, struct mdinfo *info,
 		sb->state &= ~(1<<MD_SB_BITMAP_PRESENT);
 	} else if (strcmp(update, "_reshape_progress")==0)
 		sb->reshape_position = info->reshape_progress;
+	else if (strcmp(update, "writemostly")==0)
+		sb->state |= (1<<MD_DISK_WRITEMOSTLY);
+	else if (strcmp(update, "readwrite")==0)
+		sb->state &= ~(1<<MD_DISK_WRITEMOSTLY);
 	else
 		rv = -1;
 
@@ -688,6 +692,8 @@ static int add_to_super0(struct supertype *st, mdu_disk_info_t *dinfo,
 	dk->minor = dinfo->minor;
 	dk->raid_disk = dinfo->raid_disk;
 	dk->state = dinfo->state;
+	/* In case our source disk was writemostly, don't copy that bit */
+	dk->state &= ~(1<<MD_DISK_WRITEMOSTLY);
 
 	sb->this_disk = sb->disks[dinfo->number];
 	sb->sb_csum = calc_sb0_csum(sb);
diff --git a/super1.c b/super1.c
index 35e92a3..0cd4124 100644
--- a/super1.c
+++ b/super1.c
@@ -803,6 +803,10 @@ static int update_super1(struct supertype *st, struct mdinfo *info,
 		       __le64_to_cpu(sb->data_size));
 	} else if (strcmp(update, "_reshape_progress")==0)
 		sb->reshape_position = __cpu_to_le64(info->reshape_progress);
+	else if (strcmp(update, "writemostly")==0)
+		sb->devflags |= WriteMostly1;
+	else if (strcmp(update, "readwrite")==0)
+		sb->devflags &= ~WriteMostly1;
 	else
 		rv = -1;
 
@@ -923,6 +927,7 @@ static int add_to_super1(struct supertype *st, mdu_disk_info_t *dk,
 		sb->max_dev = __cpu_to_le32(dk->number+1);
 
 	sb->dev_number = __cpu_to_le32(dk->number);
+	sb->devflags = 0; /* don't copy another disks flags */
 	sb->sb_csum = calc_sb_1_csum(sb);
 
 	dip = (struct devinfo **)&st->info;