diff -up mdadm-3.2.2/Manage.c.writemostly mdadm-3.2.2/Manage.c
--- mdadm-3.2.2/Manage.c.writemostly	2011-06-13 22:50:01.000000000 -0400
+++ mdadm-3.2.2/Manage.c	2011-07-27 14:12:18.629889841 -0400
@@ -741,11 +741,24 @@ 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 -up mdadm-3.2.2/mdadm.h.writemostly mdadm-3.2.2/mdadm.h
--- mdadm-3.2.2/mdadm.h.writemostly	2011-07-27 14:12:28.800779575 -0400
+++ mdadm-3.2.2/mdadm.h	2011-07-27 14:04:34.669932148 -0400
@@ -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 -up mdadm-3.2.2/super0.c.writemostly mdadm-3.2.2/super0.c
--- mdadm-3.2.2/super0.c.writemostly	2011-06-17 01:15:50.000000000 -0400
+++ mdadm-3.2.2/super0.c	2011-07-27 14:12:18.655889559 -0400
@@ -570,6 +570,10 @@ static int update_super0(struct supertyp
 		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 supertyp
 	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 -up mdadm-3.2.2/super1.c.writemostly mdadm-3.2.2/super1.c
--- mdadm-3.2.2/super1.c.writemostly	2011-06-17 01:15:50.000000000 -0400
+++ mdadm-3.2.2/super1.c	2011-07-27 14:12:18.656889548 -0400
@@ -803,6 +803,10 @@ static int update_super1(struct supertyp
 		       __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 supertyp
 		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;