Ever thought about creating an Image of your data onto an external drive? Works for me every time I migrate controllers or change platforms. What you need is your data, so starting fresh with new parameters native to the new controller is always easiest.
Other than that, consult the sticky by SomeJoe, he has a very useful thread on the subject.
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.