I've got a test server which was running WS 2008 Enterprise on the bare metal. It was correctly using the software RAID 1 configuration (2x250 GB disks which appeared as one disk), setup on the Dell Poweredge T110 (which meets compatibility requirements) without requiring any extra setup from me. (As an aside I'm fairly sure it's software RAID, as we didn't spec a hardware RAID controller, if that's of any importance in this situation).
I am now testing installing ESXi 5.0 on this server to run some VMs. I've successfully installed ESXi, and imported a VM fine, but it's showing 2 x 250 GB disks available as datastores. However they should be appearing as one volume. When I boot the server, there is a RAID configuration screen you can enter, and I'm guessing this is what I'll have to do at some stage, but now need to be very careful because there is one disk which contains data that I want to be mirrored on the other disk. What is the best thing to do in this situation?