You might want to read up on the different RAID types before you do anything. Ideally you want both drives to be the same size, identical drives from the same manufacturer being optimal. If you are going to mirror, RAID 1, you should be able to do it without loosing data. I'd back up anyway just in case. If you are going to stripe ,RAID 0, you'll probably going to have to do a clean install. I would anyway. I've never done software RAID, only hardware RAID, and its been a while so times may have changed, but I always ended up doing a clean install when setting up my RAID array.