Joe, I'll try that block size you recommend. The lenght of time it will take is of secondary importance. My wife does not understand why we can't watch a movie while I'm expanding the capacity of the server. She just wants it to work without other fuss around.
I know I'm limited by the PCI bus, but I don't feel like spending 350€ on new hardware when all I do is usenet and watch movies from the server. I'd prefer to spend that money in buying new disks, that's 6TB I can buy for that amount. I have 2 onboard sata ports and they are being used by the parity and the cache drive. Thanks for the advice, I already considered it.