Well the plan was to create an additional new RAID 5 on the Promise card. If possible to create a mirror externally why can't I just mirror right onto the new RAID 5 on the card. I was under the impression that that wouldn't work for some reason.
In short I have a 3 drive RAID with 3 x 250 on the MB. I woudl like to get 3 X 250, or larger, and the Promise card. Create the new RAID and get the operating system and other data to the newly created RAID. Maybe it's as simple as creating a mirror, but that would be to easy.