They we clean/new disks. When I attach 1 disk there's no problem at all.
I also did a setup with 1 500GB and 1 1,5 GB disk and gives the same "message"...

Is there a possibility that it's a "windows" RAID-controller? So it needs software to run to make it a RAID1-array? (like the old days with win-modems).

I set in the BIOS of the RAID-controller to RAID1, looks to me that this should be enough...