As we know, each hard drive counts its power-on count and power-on
hours. Why these interesting values aren't counted also by mainboards?
Where would this information be stored exactly? The information you are asking about is stored in a cache within the physical hardware of the HDD. This information follows a semi-standard called
S.M.A.R.T it is useful dianostic information.
It would be nice to look at all those old computers I have at home a
see how many operating hours they achieved.
Unless the storage devices in question supported S.M.A.R.T this is not possible.
Or do at least Windows count it?
Most operating systems can only keep track of how long they were ran without a restart. You can figure out the lifetime of the system by finding the oldest file on the storage device.
I know that there probably is some third party software to do that,
but I don't consider it reliable, because during the long life of a
computer I usually change the operating system and whole disk a few
So even if the operating system tracked this information, it would be lost, because you would have wiped the information.