Hard drives are mechanical devices and susceptible to many kinds of failure.

• manufacturing defects
• mishandling during shipping or installation
• EMI, magnetic fields, static electricity
• operating environment - power fluctuations, temperature, vibration, shocks, bad software
• wear and tear - seeking, spinning, on/off cycling

Striping drives significantly decreases reliability, more so than commonly expected. Striped arrays should not be used to store important information because there is a very high probability of failure. On the other hand, mirroring drives significantly increases reliability, and a two drive mirrored array will tend to survive a long time. Precise calculations of reliability in multi-drive arrays is complex enough that teams at IBM research it, and an alternative would be a policy of preparing for the worst and then deriving reliability information from experience in the particular environment at hand.

For more information, search for Hard Disk Reliability in Google. In the example at the end of the article, the three disk RAID-0 array is 10,000 times more likely to fail in a given month than the three disk RAID-1 array. In fact, the odds of the three disk RAID-0 array failing in a given month are an incredible one in twenty. Remember that other parts of the system can fail besides just the drives, including the controllers and the cables themselves.