Good News!!
I used the drive from my PC and was able to successfully install OMV!!
I thank you very much for your help! You can´t imagine how happy I am to have a functional Server!
In Addition I think I found the reason for this bug. When I tried to install OMV via CD the first time I forgot the 3TB Data Disk in the hot-swap-case. I noticed this during the installation when I was supposed to choose the OS-disk. I selected the right one and continued. But I also noticed that the 3 TB disk was "sda" and the 1 TB disk I wanted to choose was "sdb". This got interesting when it came to the grub installation because the installer tried to install grub to the 3 TB disk alias "sda" and failed. In this moment I remembered that the bootstick which I used before was as well identified as "sda".
So in conclusion I think the reason for this bug was that the installer always installed grub to "sda" no matter if this was the OS-disk
I don´t know if this was just the case for me but if this is a bug in the installer this can be maybe fixed.
overgrateful
Jaster