Seemed to be the only way bearcat - but not good to have to disable the whole 6 port SATA controller (2x6Gbps & 4x3Gbps) to disable only 2 ports. I have now arranged purchase of a new LSI2308 8 port SAS PCI-E and a SFF-8087 SAS to 4 port SATA cable to make up for the lost ports.
System is now running with 8 disks (RAID 6) until I get/add the new hardware.