I've tried some other things, but without success. Eventually I have recovered the files on the SSD, formatted it and re-installed Windows. Before installation, I also connected the four RAID drives to the mobo. But even after installing all the drivers, Windows doesn't recognize the RAID array properly.
Windows Device Manager shows "Intel Raid 1 Volume SCSI Disk Device" as first item. Then the SSD followed by two times "ST2000DM 001-1CH164 SCSI Disk Device".
What can I do to fix this (without losing data)?