In the "mirroring" mode, dont you lose space and speed? Whats the difference between having that and a backup hdd?
Yes you lose space, but not speed. I've read you may actually have a slight gain in reed speed, but that's irrelevant. The difference is it's a mirror, not a backup, so if you delete something it is deleted from both hdd's. However, you have the benifit of being able to replace a drive if it dies while still using the computer. It will be slower while the RAID arrays rebuilds, but in a business the lack of downtime is important.
@RoadStress: I don't mean to ignore you, I just don't know the answer to your question. I would love to see a seperate topic started on this. So if you start one, please link to it in this thread or send me a pm.