mdadm device missing

Question

after a reboot my /dev/md0 device was missing. This is a RAID-1 array from two partitions of two harddrives. On each of the harddrives is another partition, which together belong to dev/md1. However, only dev/md0 was missing.

cat /proc/mdstat and mdadm --detail --scan revealed both only one array (md1). But mdadm -Es showed up both arrays:

ARRAY /dev/md0 level=raid1 num-devices=2 UUID=xx
ARRAY /dev/md1 level=raid1 num-devices=2 UUID=xx

Has anyone an idea why /dev/md0 is not mapped and not identified correctly?

Answer 1

To get my /dev/md0 array back I added the output of mdadm -Es to my /etc/mdadm/mdadm.conf. Before there was only the automatic scan with DEVICE partitions configured.
However I am still curious why the automatic scan misses the one array.