There is no hardware RAID solution that I know of that will do what you want. If it can be done, it would have to be in software. I know Windows server (NT/2K/2K3) do RAID-5 in software, and all Linux ...
I haven't had an external drive since I stopped using my SCSI tower about five years ago. Since then I've tolerated the acceptable speeds of IDE for the tradeoff in large amounts of storage. As my box ...