Nope no artificial limit...
Check to verify whether the 250GB isn't connected to the SATA 6Gb port on the board. If so, perhaps you need newer drivers for the SATA controller to pick up the last drive.
Only the 500GB drive on which OS is installed supports 6GB/s. And it is connected to the only 6gbps port on the board. The OS detects only two SATA ports, i think. I disconnected 1TB drive and connected the 250 GB to that port and it worked.
Also, just checked asus website and my drivers are uptodate.