Hard disk power on time can be determined by examining the #9 "Power on time count" S.M.A.R.T. attribute of the hard disk. This value is supported by most modern IDE and S-ATA hard disks.
The raw value of this attribute is constantly increasing when the hard disk is under power (running, but not neccessarily performing a read or write operation). The unit of the time depends on the manufacturer. It can be hours (the value is increased once per every hour), minutes or half-minutes. Different units are also possible and two hard disks from the same manufacturer may not have same time units.
After installation, Hard Disk Sentinel starts examining this attribute. It checks the elapsed time between two consecutive increases of the raw value of this attribute. During this test, an estimated power on time value is displayed (with grey color) and it may not be accurate. This test can run up to 20 minutes. If the user closes the application during the test, it will be automatically restarted on the next launch of Hard Disk Sentinel until it can determine the unit of time used on all hard disks. After successful measure, the unit is saved and it will be used automatically later. This measurement has no effect on the performance of the hard disk or the computer.
After the measurement (or if it is not required) the exact, accurate power on time value is displayed (with black color) and the "estimated value" text is not displayed. If the displayed value seems to be incorrect (too low or high), the "Repeat Test" button can be used.
When the measurement completed, it is recommended to send a test report. The received reports help in further development and it is possible that in a later version the exact power on time count will be displayed from the first time (without the need of the measurement).
The value displayed next to the Power on time indicates the total time the drive was running (powered). Idle time (when there was no read or write operation) also increases this value but standby time does not increase this time. One displayed day means 24 hours. For example, a two years old hard disk which is used 12 hours every day will show the same "power on time" as a one year old hard disk which is used in 24/7 mode (like a 3 years old hard disk working 8 hours every day).
Note: the power on time display may not be correct on some models.
By default, the total expected lifetime of a hard disk in perfect condition is defined as 5 years (running every day and night on all days). This is equal to 1825 days in 24/7 mode or 43800 hours.
The elapsed power on time decreases this value.
The result is decreased according the number of days when the temperature was too high. If the maximum (peak) temperature was high, it is not so critical but if the average temperature was too high (it means the temperature was high for longer time) the result is reduced drastically.
The result is multiplied by the square of the health value of the disk.
Note: Hard Disk Sentinel displays 1000 days as maximum value for estimated remaining lifetime. Of course in an ideal situation this value will not decrease even after hundreds of days.
For example: the hard disk is working for 600 days (600 x 24 hours) and its temperature was not too hot (there were no days with too high average temperature) and the health is 70%. The total remaining lifetime is 1225 x 0.7 x 0.7 = 600 days and 6 hours.
As you can see, the health value drastically determines the estimated remaining lifetime of the hard disk. If the health value is low, the remaining lifetime display will remain unchanged for many days, even if the power on time value increases. For example, if the health value is 25% for a hard disk, 16 days needed (assuming the hard disk is working 24 hours daily) to decrease the remaining lifetime with one single day (because 16 x 0.25 x 0.25 = 1).
Note: the estimated remaining lifetime depends on many factors (temperature, health) so it is recommended to perform the steps of the temperature calibration and select the correct condition calculation method based on the actual situation and use the correct offset values for the S.M.A.R.T. attributes if neccessary. Only advanced users should use and modify these options because a wrong combination of settings may result in improper display of some values.
If the "Analyse vendor specific values" setting is used, the estimated lifetime is determined by the threshold exceed date for the critical S.M.A.R.T. attribute which will reach its threshold before any other critical S.M.A.R.T. attributes of the hard disk. Such threshold exceed dates are displayed on the S.M.A.R.T. attribute graphs of a hard disk (if the "display vendor specific values" option is selected next to the graph).
Note: this method is reliable only if Hard Disk Sentinel constantly analyses the hard disks for a very long time (this may be weeks or even months). In other cases it is possible to predict incorrect results by a small degradation of the attributes (for example, threshold exceed date would be in January 2100. Of course this is not real but such values may appear in other hard disk monitoring applications). To avoid this situation, Hard Disk Sentinel displays 1000 days as maximum value for estimated remaining lifetime. Of course in an ideal situation this value will not decrease even after hundreds of days.