SMART (Self Monitoring, Analysis and Reporting Technology) is a hard disk monitoring system that is built into the HDD logic boards. It provides a host of parameters that are actively monitored by the HDD during its normal operation. Note, this monitoring and recording of information does not require any support from the Operating System.
The article is from Google below analyzes SMART parameters and their correlation to HDD failure.
They found that the parameters below have a large impact on failure probability.
Scan Error (No SMART Parameter)
Drives typically scan the disk surface in the background and report errors as they discover them. Large scan error counts can be indicative of surface defects, and therefore are believed to be indicative of lower reliability.
Reallocation counts (SMART Parameter: 0x05)
When the drive’s logic believes that a sector is damaged (typically as a result of recurring soft errors or a hard error) it can remap the faulty sector number to a new physical sector drawn from a pool of spares. Reallocation counts reﬂect the number of times this has happened, and is seen as an indication of drive surface wear.
Ofﬂine reallocation counts (SMART Parameter: 0xC4, 0xC6)
Ofﬂine reallocations are defined as a subset of the reallocation counts studied previously, in which only reallocated sectors found during background scrubbing are counted. In other words, it should exclude sectors that are reallocated as a result of errors found during actual I/O operations. Although this definition mostly holds, we see evidence that certain disk models do not implement this definition.
Probational counts (SMART Parameter: 0xC5)
Disk drives put suspect bad sectors “on probation” until they either fail permanently and are reallocated or continue to work without problems. Probational counts, therefore, can be seen as a softer error indication. It could provide earlier warning of possible problems but might also be a weaker signal, in that sectors on probation may indeed never be reallocated.
I use Crystal Disk Info on Windows 7 to report on some of the parameters above.