<P> While RAID may protect against physical drive failure, the data is still exposed to operator, software, hardware, and virus destruction . Many studies cite operator fault as the most common source of malfunction, such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process . </P> <P> An array can be overwhelmed by catastrophic failure that exceeds its recovery capacity and the entire array is at risk of physical damage by fire, natural disaster, and human forces, while backups can be stored off site . An array is also vulnerable to controller failure because it is not always possible to migrate it to a new, different controller without data loss . </P> <P> In practice, the drives are often the same age (with similar wear) and subject to the same environment . Since many drive failures are due to mechanical issues (which are more likely on older drives), this violates the assumptions of independent, identical rate of failure amongst drives; failures are in fact statistically correlated . In practice, the chances for a second failure before the first has been recovered (causing data loss) are higher than the chances for random failures . In a study of about 100,000 drives, the probability of two drives in the same cluster failing within one hour was four times larger than predicted by the exponential statistical distribution--which characterizes processes in which events occur continuously and independently at a constant average rate . The probability of two failures in the same 10 - hour period was twice as large as predicted by an exponential distribution . </P> <P> Unrecoverable read errors (URE) present as sector read failures, also known as latent sector errors (LSE). The associated media assessment measure, unrecoverable bit error (UBE) rate, is typically guaranteed to be less than one bit in 10 for enterprise - class drives (SCSI, FC, SAS or SATA), and less than one bit in 10 for desktop - class drives (IDE / ATA / PATA or SATA). Increasing drive capacities and large RAID 5 instances have led to the maximum error rates being insufficient to guarantee a successful recovery, due to the high likelihood of such an error occurring on one or more remaining drives during a RAID set rebuild . When rebuilding, parity - based schemes such as RAID 5 are particularly prone to the effects of UREs as they affect not only the sector where they occur, but also reconstructed blocks using that sector for parity computation . Thus, an URE during a RAID 5 rebuild typically leads to a complete rebuild failure . </P>

Which raid level provides the most capacity while being able to sustain 2 drive failures