Break RAID0 to have separate drives
This isn't an issue with OS X itself, it must be the enclosure that's combining the drives. If it was a software RAID, you would see the drives as disk1 and disk2, and the combined volume as disk3. It's possible that the enclosure's software is refusing to destroy the combined volume because it's partitioned and breaking it apart would result in data loss (although in your case obviously not). I suggest deleting the partition on there and trying to separate them in the device-specific software again. If that fails, try physically taking out one of the drives, configuring the other one on its own, and only then adding the second one.

Have you tried both eSATA and USB modes to see if there is any difference? (assuming you have a spare eSATA port)

I'm unfortunately not familiar with Hornettek and its devices, so I can't make any more specific suggestions than that.