I have read somewhere what RAID is limited up to 1,3 Go.
Resolved. After spending too many days in an attempt to transfer to the new 2 TB drives I decided to start fresh. Since I had upgraded from XP to Vista I had the original Vista Upgrade DVD's. I removed one of the two original drives & install one of the new 2 TB drives. Then installed a clean version of Vista onto the new 2 TB drive. I also transferred all the data over to the new drive. For some reason the system would only work if I changed the BIOS setting to Raid Only. Then replaced the 2nd old drive with the 2nd new 2TB drive. Was able to mirror the drives. You will need to run windows update about 5 times.... reload your software, etc. Plan to spend an entire day to accomplish this.
While the Raid system was always able to boot and recover with either of the original drives, it was very disappointing that the system would not allow for a drive upgrade. Since I used a work around I don't know why it would not work. Could be Windows, Intel chip set, or perhaps BIOS issue. Don't know. But it was a very time consuming, frustrating event!
It has now been some time since I completed the upgrade. I can report that I am Very Pleased with the results. Between the fresh vista install & new WD hard drives my system performance improved significantly! It put new life into this PC.