To add to the comments of cascading failure: if a drive goes bad, another drive from the same manufacturing batch is disproportionately likely to go bad. RAID a

To add to the comments of cascading failure: if a drive goes bad, another drive from the same manufacturing batch is disproportionately likely to go bad. RAID arrays are often built with drives from the same batch, since they were bought at the same time from the same vendor. This means array failures include multiple drives more often than you'd expect.