Thanks Ben,
your suggestion works like a charm. After i have set the AHCI Capable SATA Controller to ENHANCED mode and switched the SAS/SATA Capable Controller back to INTEL(R) RSTe, the drives get successfully recognized by the hypervisor.
If you have any further suggestions on this specific hardware setup, feel free to share it with me. As i'm very new on the ESXi platform, i'm happy for any help I can get. Therefor I have to thank you both again for your good help.
Greets, Markus
PS: I think we will stick with ESXi and buy the VMWare vSphere Standard Edition in the next 2 weeks, as the evaluation went very well, although the onboard controller wasn't functional.