What is SMART?
SMART is a technology which provides hard disk drives with methods to predict certain kinds of failures with certain chance of success.
A Detailed Answer
Self-Monitoring, Analysis, and Reporting Technology, or SMART, is a monitoring system for hard drives to detect and report various indicators of reliability, in the hope of anticipating failures. SMART is implemented inside the drives, providing several ways of monitoring the drive health. It may present information about general health, various drive attributes (for example, number of unreadable sectors), error logs, and so on. It may also provide ways to instruct the drive to run various self-tests, which may report valuable information. It may even automatically scan the disk surface in when the drive is idle, repairing the defects while reallocating the data to more safe areas.
While having SMART sounds perfect, there are some nuances to consider. One of the common pitfalls is that it may create a false sense of security. That is, a perfectly good SMART data is NOT an indication that the drive won't fail the next minute. The reverse is also true - some drives may function perfectly even with not-so-good-looking SMART data. However, as studies indicate, given a large population of drives, some SMART attributes may reliably predict drive failures within up to two months.
Another common mistake is to assume that the attribute values are the real physical values, as experienced by the drive. As manufacturers do not necessarily agree on precise attribute definitions and measurement units, the exact meaning of the attributes may vary greatly across different drive models.
At present SMART is implemented individually by manufacturers. While some aspects are standardized for compatibility, others are not. In fact, most manufacturers refer the users to their own health monitoring utilities and advice against taking SMART data seriously. Nevertheless, SMART may prove an effective measure against data loss.
Yet another issue is that quite often the drives have bugs which prevent correct SMART usage. This is usually due to buggy firmware, or the manufacturer ignoring the standards. Luckily, smartmontools usually detects these bugs and works around them.