Dear David and others,

thanks a lot for so much discussion and details. I learn a lot.
Following your discussions I see there still is some basic knowledge 
missing on my side.

Am 30.11.2020 21:05 schrieb David T-G:

You don't see any "filesystem" or, more correctly, partition in your

  fdisk -l

I do not see the partition in the output of "fdisk -l".

But I can (when both discs are present) mount /dev/md127 (manualy via 
mount and via fstab) to /Daten and create files on it.

So the display isn't interesting, although the logic behind that 
approach
certainly is to me.

I plugged in the nacked hard discs and they appear as /dev/sdb and 
/dev/sdc. After that
   mdadm --create /dev/md/md0 --level=1 --raid-devices=2 /dev/sdb 
/dev/sdc
Then I did
  ls -l /dev/md/md0 and found out this is just a link to /dev/md127.
I formated the raid with
  mkdfs.ext4 /dev/md127
Then I mounted (first manually via mount and after sucess via fstab) 
/dev/md127 to /Daten

Is this unusual?