those two disks should become one md0 RAID1 mirroring array…

# list installed block devices
root@Microknoppix:/# lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 931,5G  0 disk
└─sda1   8:1    0 931,5G  0 part
sdb      8:16   0   1,8T  0 disk
sdc      8:32   0  29,8G  0 disk
├─sdc1   8:33   0     1G  0 part [SWAP]
└─sdc2   8:34   0  28,8G  0 part /
sdd      8:48   0   1,8T  0 disk
sr0     11:0    1  1024M  0 rom
sr1     11:1    1  1024M  0 rom
zram0  251:0    0   2,6G  0 disk [SWAP]

# start partitioning the first disk
fdisk /dev/sdb
# create new primary partition
n (new partition)
p (primary partition, hit enter, enter, enter to use the full disk as one big partition)
t (hit t then enter: fd followed by enter to label the partition "linux raid autodetect")
w (hit w to write changes to disk)

# ... do the same with with all your disks.

# create the raid1 array containing the two partitions
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdd1

ls -l /dev | grep sd; # list all disks
brw-rw----. 1 root disk 8, 16 Jan 15 00:02 sdb
brw-rw----. 1 root disk 8, 17 Jan 15 00:02 sdb1
brw-rw----. 1 root disk 8, 32 Jan 14 23:50 sdc
brw-rw----. 1 root disk 8, 33 Jan 14 23:50 sdc1
brw-rw----. 1 root disk 8, 48 Jan 14 23:51 sdd
brw-rw----. 1 root disk 8, 49 Jan 14 23:51 sdd1
brw-rw----. 1 root disk 8, 64 Jan 14 23:51 sde
brw-rw----. 1 root disk 8, 65 Jan 14 23:51 sde1
brw-rw----. 1 root disk 8, 80 Jan 14 23:51 sdf
brw-rw----. 1 root disk 8, 81 Jan 14 23:51 sdf1
brw-rw----. 1 root disk 8, 96 Jan 14 23:51 sdg
brw-rw----. 1 root disk 8, 97 Jan 14 23:51 sdg1
brw-rw----. 1 root disk 8, 112 Jan 15 00:02 sdh
brw-rw----. 1 root disk 8, 113 Jan 15 00:02 sdh1 # hot spare
# if you want to create RAID10
mdadm --create /dev/md0 --level=10 --raid-devices=6 /dev/sd[b-g]1

mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
Continue creating array? Y
mdadm: Defaulting to version 1.2 metadata

mdadm: array /dev/md0 started.

mdadm --add /dev/md0 /dev/sdh1; # add hot spare

watch -n 2 cat /proc/mdstat; # now you will see it build the array

Every 2.0s: cat /proc/mdstat                                                                                                         Mon Jan 15 01:38:56 2018

Personalities : [raid10]
md0 : active raid10 sdh1[6](S) sdg1[5] sdf1[4] sde1[3] sdd1[2] sdc1[1] sdb1[0]
      5860147200 blocks super 1.2 512K chunks 2 near-copies [6/6] [UUUUUU]
      [>....................]  resync =  1.7% (102443584/5860147200) finish=532.3min speed=180266K/sec
      bitmap: 44/44 pages [176KB], 65536KB chunk

# now you have an new device md0 that represents your raid1-array

# lets format this new array and label it RAID10
mkfs.ext4 -L RAID10 /dev/md0

# it's a good idea to give it a meaningful label
e2label /dev/md0 "RAID1DATA"

Add entry to /etc/fstab:

/dev/md0 /run/media/forstner/RAID10 ext4 noatime,rw 0 0

if you use this moint point CentOS MATE Desktop CAJA (File Explorer) will show it like this:

and you can mount it from the GUI by a click on RAID10. NEAT!

Create mdadm config file:

# test config output
root@Microknoppix:/home/knoppix# mdadm --examine --scan
ARRAY /dev/md/0 metadata=1.2 UUID=53ca5f4e:c16a6706:a216c510:5122e016 name=Microknoppix:0

# save config output to config file
root@Microknoppix:/home/knoppix# mdadm --examine --scan >> /etc/mdadm/mdadm.conf

Note that the array is actually started in /etc/rc0.d/K13mdadm-raid (via mdadm -A -s) or mdrun command

# test what would happen on reboot
/etc/rc0.d/K13mdadm-raid start

# sample output
[ ok ] Assembling MD array md0...done (started [2/2]).
[ ok ] Generating udev events for MD arrays...done.

# to watch your raids at work
watch -n 2 cat /proc/mdstat

source: https://wiki.debian.org/SoftwareRAID

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