Willie, maybe a backup drive would work better than a RAID array. My experience with RAID was only when setting up servers and we always started with blank drives, of course. We did expand the array, blank drives, but again and it took a significant amount of time to "re-stripe" the system. All of the ones I worked with were RAID 5 and the hardware allowed hot swaps in the event of a failure. Like Mike said, when you create a RAID array the drives get formatted to RAID, at the level you select, so anything on them is history. RAID 5, if I remember correctly, is the "safest" with the data spread/duplicated across the drives so that if one or two fail simultaneously, nothing is lost. Another thing about RAID 5 is that it has a relatively high "overhead" in terms of lost drive capacity for the striping information to be tracked. We did have a power problem, years ago, and lost three out of a five disk array and had to restore from tape, DAT2. I spent the entire weekend in the server room on that one.
I've got a 2 gig and 4 gig, USB 3.0, backup drive setup at the moment. The 2 gig filled up way too fast and I don't have a lot of video on my system which could limit usefulness for you. The first backup, a "full", did take some time, but the incrementals are very fast and the data is easy to recover if needed.