All modern hard disks are S.M.A.R.T. enabled. «SMART is a monitoring system for hard disks to detect and report on various indicators of reliability, in the hope of anticipating failures».
Most people never pay attention to the numbers, some people only pay attention to the templerature, and if something bad happens (a failure) then they give some attention to the error counters.
But here are two counters that I found very interesting: Power_On_Hours and Power_Cycle_Count.
A hard disk used in my desktop machine
A hard disk used in a server machine
To see this info about your hard disk, use «smartctl -a /dev/sda | grep Power».
smartctl is part of the smartmontools package in most linux distros.
This was triggered by the following tweet