nas ~ # mdadm --verbose --manage --set-faulty /dev/md3 /dev/sda4
mdadm: set /dev/sda4 faulty in /dev/md3
nas ~ # mdadm --verbose --manage /dev/md3 --remove /dev/sda4
mdadm: hot removed /dev/sda4 from /dev/md3

nas ~ # fdisk /dev/sda
Welcome to fdisk (util-linux 2.24).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.Command (m for help): p
Disk /dev/sda: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 0C5623E2-2FF4-4787-9609-750745C161DA

Device           Start          End   Size Type
/dev/sda1         2048      1026047   500M EFI System
/dev/sda2      1026048      2050047   500M Linux RAID
/dev/sda3      2050048    106907647    50G Linux RAID
/dev/sda4    106907648   2983721397   1.3T Linux RAID
/dev/sda5   2983723008   5860533134   1.3T Linux RAID

Command (m for help): d
Partition number (1-5, default 5):

Partition 5 has been deleted.

Command (m for help): d
Partition number (1-4, default 4):

Partition 4 has been deleted.

Command (m for help): n
Partition number (4-128, default 4):
First sector (34-5860533134, default 106907648):
Last sector, +sectors or +size{K,M,G,T,P} (106907648-5860533134, default 5860533134):

Created a new partition 4 of type 'Linux filesystem' and of size 2.7 TiB.

Command (m for help): t
Partition number (1-4, default 4):
Partition type (type L to list all types): 14

Changed type of partition 'Linux filesystem' to 'Linux RAID'.

Command (m for help): p
Disk /dev/sda: 2.7 TiB, 3000592982016 bytes, 5860533168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 0C5623E2-2FF4-4787-9609-750745C161DA

Device           Start          End   Size Type
/dev/sda1         2048      1026047   500M EFI System
/dev/sda2      1026048      2050047   500M Linux RAID
/dev/sda3      2050048    106907647    50G Linux RAID
/dev/sda4    106907648   5860533134   2.7T Linux RAID

Command (m for help): w

The partition table has been altered.
Calling ioctl() to re-read partition table.
Re-reading the partition table failed.: Device or resource busy

The kernel still uses the old table. The new table will be used at the next reboot or after you run partprobe(8) or kpartx(8).

nas ~ # partprobe
nas ~ # mdadm --verbose --manage --add /dev/md4 /dev/sda4
mdadm: added /dev/sda4
nas ~ # mdadm -D /dev/md4
/dev/md4:
        Version : 1.2
  Creation Time : Mon Dec  9 22:06:12 2013
     Raid Level : raid5
     Array Size : 5753362432 (5486.83 GiB 5891.44 GB)
  Used Dev Size : 2876681216 (2743.42 GiB 2945.72 GB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Tue Dec 10 19:38:06 2013
          State : active, degraded, recovering
 Active Devices : 2
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 512K

 Rebuild Status : 0% complete

           Name : nas:4  (local to host nas)
           UUID : f6924d42:7358df85:416f97f9:83aeac62
         Events : 5393

    Number   Major   Minor   RaidDevice State
       3       8        4        0      spare rebuilding   /dev/sda4
       1       8       68        1      active sync   /dev/sde4
       2       8       84        2      active sync   /dev/sdf4