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Is there a table of how to interpret the SMART attribute "power on hours" depending on the HDD manufacturer? Some of them use hours, some minutes or even seconds...

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Good question indeed. I wonder why someone has down-voted it! – Wizard79 Jan 29 '14 at 15:00
I've never seen a drive that used anything other than hours to represent "power on hours" you have an example? For instance I have an SSD that stores power on time in milliseconds, but it also says that it does. – Michael Hampton Jan 31 '14 at 12:11
up vote 2 down vote accepted

See this FAQ answer from the Smartmontools project:

There are links there to vendor specific pages which may provide you with the information you're looking for.

Some other relevant FAQs on the same page:

smartctl reports the age as thousands of hours for my Maxtor/Hitachi/Fujitsu? disk , yet it is only a few days old

On recent disks, Maxtor has started to use Attribute 9 to store the power-on disk lifetime in minutes rather than hours. In this case, use the: '-v 9,minutes' option to correctly display hours and minutes.

Some models of Fujitsu disks use Attribute 9 to store the power-on disk lifetime in seconds. In that case, use the: '-v 9,seconds' option to correctly display hours, minutes and seconds.

The power-on timer (Attribute 9 raw value) on my Maxtor disk acts strange.

There are three related problems with Maxtor's SMART firmware:

On some Maxtor disks, the raw value of Attribute 9 (Power On Time) is supposed to be minutes. But it advances at an unpredictable rate, always more slowly than one count per minute. This is because when the disk is in idle mode, the counter stops advancing. This is only supposed to happen in standby mode. This will be corrected in Maxtor product lines released after October 2004.

In Maxtor disks that use the raw value of Attribute 9 as a minutes counter, only two bytes (of the six available) are used to store the raw value. So it resets to zero once every 65536=216 minutes, or about once every 1092 hours. This is fixed in all Maxtor disks manufactured after July 2003, where the raw value was extended to four bytes.

In Maxtor disks that use the raw value of Attribute 9 as a minutes counter, the hour time-stamps in the self-test and ATA error logs are calculated by right shifting 6 bits. This is equivalent to dividing by 64 rather than by 60. As a result, the hour time stamps in these logs advance 7% more slowly than they should. Thus, if you do self-tests once per week at the same time, instead of the time-stamps being 168 hours apart, they are 157 hours apart. This is also fixed in all Maxtor disks manufactured after July 2003.

Power_On_Hours Attribute of my new Intel SSD reports ~890000 hours

This is a bug in Intel 330 Firmware 300i and Intel 520 Firmware 400i. The offset is ​894794 hours. See also ticket #289 and ​Intel SSD Toolbox SMART Attributes FAQ. The hours counter from Device Statistics is not affected:

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