cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 sda2[3]
      14312320 blocks super 1.0 [2/1] [_U]
      bitmap: 1/1 pages [4KB], 65536KB chunk

unused devices: <none>

mdadm /dev/md127 --add /dev/sdb2

mdadm: add new device failed for /dev/sdb2 as 2: Invalid argument

Nov 13 14:18:05 SweetHome kernel: md: sdb2 does not have a valid v1.0 superblock, not importing!
Nov 13 14:18:05 SweetHome kernel: md: sdb2 does not have a valid v1.0 superblock, not importing!
Nov 13 14:18:05 SweetHome kernel: md: md_import_device returned -22
Nov 13 14:18:05 SweetHome kernel: md: md_import_device returned -22

mdadm --query --detail /dev/md127
/dev/md127:
           Version : 1.0
     Creation Time : Wed Dec  7 13:24:43 2016
        Raid Level : raid1
        Array Size : 14312320 (13.65 GiB 14.66 GB)
     Used Dev Size : 14312320 (13.65 GiB 14.66 GB)
      Raid Devices : 2
     Total Devices : 1
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Tue Nov 13 15:05:30 2018
             State : clean, degraded
    Active Devices : 1
   Working Devices : 1
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : bitmap

              Name : sweethome:root
              UUID : 7394da5e:28ad1e5d:59c517be:df27efc9
            Events : 5701454

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       3       8        2        1      active sync   /dev/sda2

[root@SweetHome ~]# fdisk -l /dev/sda

Disk /dev/sda: 250.1 GB, 250059350016 bytes, 488397168 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
Disk label type: dos
Disk identifier: 0x023092e8

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     2103295     1050624   fd  Linux raid autodetect
/dev/sda2   *     2103296    30728191    14312448   fd  Linux raid autodetect
/dev/sda3        30728192   488397167   228834488   83  Linux

[root@SweetHome ~]# fdisk -l /dev/sdb

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes, 1953525168 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
Disk label type: dos
Disk identifier: 0x5e38a98a

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb2   *     2103296    30728191    14312448   fd  Linux raid autodetect
/dev/sdb3        37015552  1953519615   958252032   83  Linux