I have the exactly same problem like rebecca in this post (Quad Sata Kit and OpenMediaVault 5 (Raspberry Pi 4)):
There seems to be a problem with mdadm command from novatech:
sudo mdadm -Cv -l5 -c64 -n4 -pls /dev/md0 /dev/sd{a,b,c,d}
sudo mkfs.ext4 /dev/md0
Because after shutdown und reboot the system, the disk array is not visible in OMV anymore.
dmesg says, that the array is dirty:
[ 1301.629207] md/raid:md0: not clean – starting background reconstruction
[ 1301.629258] md/raid:md0: device sdc operational as raid disk 2
[ 1301.629267] md/raid:md0: device sdb operational as raid disk 1
[ 1301.629275] md/raid:md0: device sda operational as raid disk 0
[ 1301.634063] md/raid:md0: cannot start dirty degraded array.
[ 1301.695452] md/raid:md0: failed to run raid set.