Hard drives are tricky little beasts. On the one hand, cheap disks are much more prone to start operating slowly when they begin to fail, instead of transition

Hard drives are tricky little beasts. On the one hand, cheap disks are much more prone to start operating slowly when they begin to fail, instead of transitioning directly from working to not working (or setting themselves as failed). On the other hand, RAID write speeds are roughly that of the slowest drive in the set. The more drives you have, the higher the probability of a malfunctioning hard drive making the whole thing go slow. Slow drives mean I/O bottlenecks, rendering the whole server unusable. Pair both things and you have actually increased your effective downtime instead of decreasing it. Now use expensive disks (that work more reliably and either work or don't with a higher probability) and just calculate wether the price increase is lower than the cost of the downtime/slowdowns when running cheap disks. Oftentimes the price premium is worth it.