[CentOS] Slow RAID resync

OK todays problem.

I have a HP N54L Microserver running centos 6.5.

In this box I have a 3x2TB disk raid 5 array, which I am in the
process of extending to a 4x2TB raid 5 array.

I've added the new disk --> mdadm --add /dev/md0 /dev/sdb

And grown the array --> mdadm --grow /dev/md0 --raid-devices=4

Now the problem the resync speed is v slow, it refuses to rise above
5MB, in general it sits at 4M.

from looking at glances it would appear that writing to the new disk
is the bottle neck, /dev/sdb is the new disk.

Disk I/O   In/s   Out/s
md0           0       0
sda1          0       0
sda2          0      1K
sdb1  3.92M       0
sdc1      24.2M   54.7M
sdd1      11.2M   54.7M
sde1      16.3M   54.7M

I partitiioned the disk with --> parted -a optimal /dev/sdb

[root at nas ~]# parted -a optimal /dev/sdb
GNU Parted 2.1
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p
Model: ATA ST2000DM001-1E61 (scsi)
Disk /dev/sdb: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary  ntfs         raid

 There is no ntfs filesystem on the disk, I've still not worked out
how to remove that flag.

 I've followed the article here -->
http://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html
 to attempt to speed it up but no joy.

Any Ideas what I've done wrong?

 parted output

[root at nas ~]# parted -l
Model: ATA ST31000528AS (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  525MB   524MB   primary  ext4         boot
 2      525MB   1000GB  1000GB  primary               lvm


Model: ATA ST2000DM001-1E61 (scsi)
Disk /dev/sdb: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary  ntfs         raid


Model: ATA ST2000DM001-9YN1 (scsi)
Disk /dev/sdc: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary               raid


Model: ATA WDC WD25EZRS-00J (scsi)
Disk /dev/sdd: 2500GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary  ntfs         raid


Model: ATA ST2000DL001-9VT1 (scsi)
Disk /dev/sde: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary               raid