# be root
sudo su

# show all drives, make sure it shows partitions, 
# such as sda1 (sda is not enough)
lsblk
AME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sda           8:0    0 18.2T  0 disk 
└─sda1        8:1    0 18.2T  0 part 
sdb           8:16   0 18.2T  0 disk 
└─sdb1        8:17   0 18.2T  0 part 
sdc           8:32   0 18.2T  0 disk 
└─sdc1        8:33   0 18.2T  0 part 
sdd           8:48   0 18.2T  0 disk 
└─sdd1        8:49   0 18.2T  0 part 
sde           8:64   0 18.2T  0 disk 
└─sde1        8:65   0 18.2T  0 part 
mmcblk0     179:0    0  7.4G  0 disk 
├─mmcblk0p1 179:1    0  512M  0 part /boot/firmware
└─mmcblk0p2 179:2    0  6.9G  0 part /

# create the array
mdadm --create /dev/md5 --level=5 --raid-devices=5 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md5 started.

# check the array
mdadm --detail /dev/md5

/dev/md5:
           Version : 1.2
     Creation Time : Sat Jun 15 17:52:00 2024
        Raid Level : raid5
        Array Size : 78126764032 (72.76 TiB 80.00 TB)
     Used Dev Size : 19531691008 (18.19 TiB 20.00 TB)
      Raid Devices : 5
     Total Devices : 5
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Sat Jun 15 17:53:11 2024
             State : clean, degraded, recovering 
    Active Devices : 4
   Working Devices : 5
    Failed Devices : 0
     Spare Devices : 1

            Layout : left-symmetric
        Chunk Size : 512K

Consistency Policy : bitmap

    Rebuild Status : 0% complete

              Name : nas7:5  (local to host nas7)
              UUID : 762dfe27:e976534f:0d66ec1b:271b9185
            Events : 16

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1
       3       8       49        3      active sync   /dev/sdd1
       5       8       65        4      spare rebuilding   /dev/sde1


# Notice that the spare drive, /dev/sde1, is shown to be rebuilding. 

# If it's a new array, format it. 
# If you format the array, this will DESTROY all your existing data!
mkfs.ext4 -F /dev/md5

# this will take forever. Let it finish.