Hard disk drives inside computers have one or more physical disks. The read/write head which is flying above this disk performs all read, write, delete operations.
As hard disk capacity increases, data stored on smaller and smaller areas on the disk surface. Any small (even microscopic) damage or dirt on the surface would highly reduce the chances to access important data. Disk drive internals are protected to keep dirt outside the disk chassis.
Accessing data sectors is not always completed without problems. It is possible that some areas of the disk surface is not readable or writable because of a small damage, scratches or dirt (even caused by a mechanical shock/damage, resulting small particless inside the disk drive). In this case, the hard disk marks this area as bad and it will never use these sectors again for any (read or write) operation. If this area previously had data, the hard disk tries to read and copy the data to a special spare area first. This procedure called reallocation. After the reallocation is completed, all access to the old area will be redirected to the spare area. The original (problematic) area of the disk cannot be accessed even after a complete reformat.
It sounds good - because the problem is automatically solved but the user will not notice anything about the constantly decreasing hard disk status until he will need to face an unusable hard disk and partial or complete data loss.
There are other possible problems can degrade the hard disk condition. High temperature, incorrect power or mechanical shock, vibration can seriously damage the hard disk. Many of such problems can be detected by using Hard Disk Sentinel and the data loss caused by the problems can be avoided.
S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology) introduced by IBM. It was created to monitor the disk status by using various methods and devices. A single ATA hard disk may have up to 30 such measured values (attributes). Some of them directly or indirectly affect hard disk health status and others have statistical information like power on time, start/stop count, temperature and so.
Today all modern IDE/Serial ATA/SCSI/SAS/USB hard disks (and SSDs too) have S.M.A.R.T. feature. It is not really a standard - so the meaning of the attributes may be different from manufacturer to manufacturer. Because of this, a manufacturer-independent disk health analysing method is neccessary to determine the real status of any hard disk. Hard Disk Sentinel designed this way, please see the condition calculation methods section.
Many attributes are used by all manufacturers and they are used in the same (or near same) way. That's why for example it is possible to detect the temperature and the power on time of many hard disks. These values (and many others) are monitored and reported by Hard Disk Sentinel software.
According the S.M.A.R.T. specifications, when the amount of errors is high enough to reach the error-level threshold, the hard disk should work for at least 24 hours to perform the data backup. But in many cases this time is not enough so it is important to recognize problems and prepare before it's too late. Hard Disk Sentinel shows 0 (zero) % health and suggests immediate replacement of the disk drive when the error-level threshold reached. The problem is that many drives fail completely (or "just" data corruption / data loss happens) long before that. This is why it is important to monitor the status and do not wait this point, when the computer BIOS may also report disk problem on power on and also Windows may show disk problems. Usually this point is too late for a complete backup.
Note: some hard disk controllers or drivers may prevent reading and displaying of S.M.A.R.T. values. In these cases, the current health, temperature or other status cannot be determined for the affected hard disk drive(s). If this happens, the Support -> Driver Zone page on the website may be useful: designed to provide driver/firmware update for many devices, disk controllers which allow detection of disk status information. Using Report menu -> Send test report to developer option in Hard Disk Sentinel can help, as this way it is possible to check and advise about the possibilities. Also you may contact the manufacturer of the controller for an updated driver or firmware, especially if you use an external disk enclosure, RAID controller or similar. Hard Disk Sentinel has highest possible compatibility with these devices too (supporting dozens of RAID controllers from all major manufacturers), but sometimes it may be not possible to determine the S.M.A.R.T. status when such devices used.
SSDs (and generally flash-based storage in memory cards, pendrives) work differently than hard disks: the memory cells in these devices experience wear during each write operations and each cells tolerate only a limited number of overwrite passes.
The "wear-leveling" feature of the SSD tries to hide/minimise this effect but generally the health of the SSD is (even if it has no problems at all) slowly but surely decreasing. Most SSD devices report the overall health of the memory cells by various attributes: if there are no (other) problems found, Hard Disk Sentinel reads these attributes to determine the complete health of the solid state device.
This is completely independent from software, Windows, restart, partitioning, formatting and so: as the amount of written data increases, the health slowly and surely decreases.
Some SSDs may have temperature sensors too (to provide temperature) but this may be not true for all models, so while health can be displayed, the temperature may be missing in the software if the SSD does not have temperature sensor and does not support temperature readout. When the S.M.A.R.T. page selected in Hard Disk Sentinel, showing the status information detected, there may be no "Temperature" attribute listed there.