OK, I think that the main problem is you're using two different speed hard drives (ATA100 & ATA66), and you may be running into data flow conflicts because of it.
What you need to do is run that floppy disk that came with your WD drive and use the ATA Management Utility to set the speed of the ATA100 drive down to ATA66 (UDMA mode 4). It's a pretty simple procedure, and will not endanger any of the info on your drives.
It's either that or get yourself another ATA100 30Gb WD to create the RAID array.