Indeed the TX2300 is basically a SATA controller that comes with special software drivers that handle the RAID part...

The TX2300 probably wrote something to the drives when you created and initialized the mirror and that is why they are still detected as fake RAID even if you plug them directly to the motherboard... Did you deleted the mirror before unplugging the drives from the TX2300?

You will need to clean the partition table on the drives first to get rid of that error...