So finally this topic is solved by an update of the BIOS, which finally gave me the acceleration button.
What in the end sounds easy was a looong way of testing.
To clarify a few things and help everybody here:
1) It is not necessary to have two HDD in RAID as said before. ISRT work on a single HDD with a SSD.
2) It is possible to have a SSD as the boot drive with the OS installed and have a secondary HDD accelerated by another SSD
3) The system BIOS has to be set to RAID mode
4) You do not have to re-intall the OS if you initially installed in IDE or AHCI mode. You can switch to RAID mode by a registry hack.
Hope that helps.