if anyone is interested in the solution.

After talking and sending lots of mails to the support, there may be a bug in the software of DSSv6.
ATM there's no way to build up bigger S/W Raids then 2TB.

e.g.
if you got 2 H/W Raid controllers with 2 harddisks on each controller. Each hdd got 2TB, and you build up a Raid0. (Got about 4TB on each H/W controller)

-> if you want to mirror these two H/W controllers with a S/W raid1, you cant to that, cause the S/W Raid exceed the limit of 2TB...
short: you cant mirror any volumes with S/W Raid1 > 2TB at the moment