What OS and version?
If teh BIOS is seeing the RAID and the OS does not , it is usually a sign that your OS does not have a native driver for the raid controller and you need to load one.
Which controller (onboard SATA, or RS2BL080) are you trying to use with the backplane? As far as I know you can't connect two controllers to one backplane. If you want to use both, connect the backplane to RS2BL080 (you may need a second cable to connect all 6 ports on the backplane), and connect SATA drives to onboard SATA ports directly using SATA cables, or using a second backplane.