I think that is your answer. Whilst it is perfectly possible to have a RAID array on internal disks, either connected to the motherboard or an add-on card, it is not possible to have such an array if connected externally via separate USB ports. You can via separate eSATA ports, since they are basically the same as the on-board ports except external. You can have an external USB RAID, however the disks need to be in a single enclosure which can utilise RAID, i.e. has hardware support, and which only requires a single USB port.