In practice, RAID 5 is risky because about half the time that a drive fails a second drive fails before the rebuild is complete. The simple mathematical models don't reflect problems which tend to affect more than one drive including problems with operating system, controller, motherboard, cables, PS/U, UPS, power cord, wall current, HVAC, etc. Years ago we used RAID 5 a lot but these days we use LVM over RAID 1. --Mike Bird