>>Yes, a mirrored (RAID1) HD is (or at least appears to be) a completely "natural" hard drive as regards format.
>>I installed mine in a USB enclosure, powered it up and attached the USB cable with some trepidation. But to my great pleasure I was able to see the entire directory structure and copy from it at will. Some .zip extractions, Word document opens, Help file opens later from the copied stuff I was convinced that all seems perfectly normal with the HD.
>
>That was my guess. Is the computer back up and running? If so what was the problem?

Let me guess - the first motherboard was reading the mirrored version of the disk in reverse, trying to rectify the mirrored image. Which was probably wrong, it's already reversed once in the controller.