I removed the RAID-controller and attached the disks to the 2 SATA-connections on the main-board. This gives the same result and my main-board doesn't support RAID at all!!!
The warning says: "Fake RAID detected on /dev/sdx. Fake RAID is not supported - please use software RAID instead." I don't use any RAID settings. Can I safely make a S/W RAID1 ?