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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Quality and Reliability | Hard Disk Quality and Reliability Specifications ]
Ack! Hard disk errors! Surely, "this is the stuff that PC nightmares are made of". :^) Fortunately, true errors are rarely encountered when using hard disks. To help users understand better the rate at which errors will occur with a hard disk, manufacturers provide anywhere from one to several error rate specifications.
The most common error rate spec is the drive's unrecoverable error rate, which is usually specified as "<1 in 10N bits", where "N" is usually between 12 and 15. "Unrecoverable" means that the drive is unable to use its error-correcting code, retries or other techniques to recover from the error in reading the disk and thus properly recreate the data. If "N" is 14, then that means this will occur every 100 trillion (100,000,000,000,000) bits read from the disk. Not too shabby. :^)
In fact, drives usually have several different error rate specifications; they are just usually not put into the short data sheets commonly distributed by hard disk manufacturers. In fairness, unrecoverable errors are the most important ones, but there are also specifications for recoverable errors, errors recovered after multiple reads and so on. To find these, you generally need to download the product manual for the drive or contact the manufacturer's technical support department. For a full explanation of these various errors and what they mean, see this full discussion of errors and error recovery.
Error rate specifications are typically used to compare drives. Within the same general class of drive there are usually relatively few differences between manufacturers. The biggest difference in error rate can be seen by comparing newer drives to older ones--newer drives are usually significantly better despite pushing the design envelope with higher speed and much greater capacity.
Next: Warranty Length