~# cat /etc/os-release
...
Ubuntu 18.04.3 LTS
...

# cat /dev | grep sd
sdc
sdd

# fdisk -l /dev/sdc
Disk /dev/sdc: 1.8 TiB, 2000398934016 bytes, 3907029168 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

# fdisk /dev/sdc

Welcome to fdisk (util-linux 2.31.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x55b821f4.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1):  
First sector (2048-3907029167, default 2048): 
Last sector, +sectors or +size{K,M,G,T,P} (2048-3907029167, default 3907029167): 

Created a new partition 1 of type 'Linux' and of size 1.8 TiB.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

# fdisk -l /dev/sdc
Disk /dev/sdc: 1.8 TiB, 2000398934016 bytes, 3907029168 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: dos
Disk identifier: 0x55b821f4

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdc1        2048 3907029167 3907027120  1.8T 83 Linux

# cat /dev | grep sd
sdc
sdc1
sdd
sdd1

# fdisk -l
Disk /dev/md0: 976 MiB, 1023410176 bytes, 1998848 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 /dev/md1: 446.1 GiB, 478941282304 bytes, 935432192 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

# mdadm --create --verbose /dev/md2 --level=1 --raid-devices=2 /dev/sdc1 /dev/sdd1
...
mdadm: size set to 1953381440K
mdadm: automatically enabling write-intent bitmap on large array
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md2 started.

# cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md2 : active raid1 sdd1[1] sdc1[0]
      1953381440 blocks super 1.2 [2/2] [UU]
      [>....................]  resync =  3.5% (69487232/1953381440) finish=166.9min speed=188113K/sec
      bitmap: 15/15 pages [60KB], 65536KB chunk

# mkfs.ext4 /dev/md2

ARRAY /dev/md/2  metadata=1.2 UUID=XXXXXXXX:XXXXXXXX:XXXXXXXX:XXXXXXXX name=HOSTNAME:2

# mdadm -D /dev/md2
/dev/md2:
           Version : 1.2
     Creation Time : Mon Dec  2 20:59:12 2019
        Raid Level : raid1
        Array Size : 1953381440 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953381440 (1862.89 GiB 2000.26 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Mon Dec  2 23:16:04 2019
             State : active, resyncing 
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : bitmap

     Resync Status : 51% complete

              Name : HOSTNAME:2  (local to host HOSTNAME)
              UUID : XXXXXXXX:XXXXXXXX:XXXXXXXX:XXXXXXXX
            Events : 4611

    Number   Major   Minor   RaidDevice State
       0       8       33        0      active sync   /dev/sdc1
       1       8       49        1      active sync   /dev/sdd1

/dev/md2      /md2     ext4    defaults    1 2

# mount /dev/md2 /md2