I would be remiss if I didn't at least TRY to help v2d at this point...
I assume the RAID cage has at least one drive in it, i.e. the one that it comes with? If so then there is no need to configure anything... just drop in your existing drive and let the mirrorring process happen. Consult the BCM installer documentation for details. The capacity of the hard drive doesn't matter, the RAID process will create paritions to match the source drive, so only 20 GB will get used.