i found it out by myself at least.
The thing was that you can't select "Raid 0+1" or so, only "security" (means mirroring) or "performance" (means striping). I had made a data backup from my stripe and tried to create the Raid 0+1.
At first i had created 2 arrays as stripe with 2 HDD in every array and tried to mirror the 1st with the 2nd. For that I tried to create a 3rd array and would put-in the two arrays that i had created before. No way... i got the error message "No disk available"
Then i tried the other way and had created 2 arrays as mirror and would stripe them. But same result like before...
And then, after hours and hours of thinking what i make wrong, a asked to me what happens, if i select "security", create only ONE array and put ALL 4 disk into it.And that's it!!!!
Directly after all 4 HDD were assigned to this one array the state of the Raid was changing from "security" to "security/performance" and voila i had my Raid 0+1.
The system was complete new installed then and for additional security i've made an image from the boot partition.
Now all works fine and i can agree with Marty Sems (http://www.smartcomputing.com/Editorial/article.asp?article=articles/2004/w1510/13w02/13w02.asp&guid
) that the Raid 0+1 has nearly the same performance like the Raid 0.
So I'm happy now that all works fine and that i have much more data security.
Many thanks for all your help!