...sigh.
Why RAID 5 stops working in 2009 | Storage Bits | ZDNet.com
It's not about usable space, it's about physical capacities. The higher you go
overall, the more likely you are to see a random error that hoses the whole array.
RAID 5 is slower than RAID 0. Always. RAID 5 is equivalent to one drive usually, no matter how many spindles. They all still have to read and write together. I've run RAID 5 on 10K SCSI drives, still wasn't fast. True hardware RAID, mind you.
I have had a parity error, the one the article mentioned. It never made sense to me why it happened until I read that. That's why I use a spare single drive that matches or is larger than the array as a secondary backup. The array can freak and I'm fine. The controller can freak and I'm fine. The single drive can die and I'm still fine. It pays to know these things before you think you're safe
No need for 2 controllers unless you're in an enterprise environment. Just use the single as secondary and you'll be fine. That is, unless you want to go 2TB+.