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I have 8TB HDD WD Gold. Here they claim 2M MTBF (mean time before failure) hours. What is the statistical variance (estimated at least) of this data ?

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MTBF is often based on an exponential distribution model exp(-lambda*T). The MTBF is equal to 1/lambda. The variance can be calculated, it is 1/lambda², that is a standard deviation also equal to 1/lamdba (hence equal to the MTBF itself).

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    Now the point is, the variance brings little useful information: -- it is fully dependent on the assumed distribution and not independent from the MTBF (the exponential model is described by a single parameter) -- the maximum of the distribution is always at T=0 and not at all near the MTBF value. Some vendors may use other models than the simple exponential one, but which are not very different from it. The exponential model is based on a constant failure rate over time, which is close to what happens in the real world.
    – PierU
    Commented Aug 29, 2022 at 14:12
  • The best for me would be if you could roughly plot the graph of WD 8TB Gold, First Failure. On the "x" axis should be hours of up HDD and on "y" probability that it fails in a given interval on 'x". So something like density f.
    – user865044
    Commented Aug 29, 2022 at 18:18
  • For x expressed in years and y the probability (in %), this is the curve: prnt.sc/xaM7OX3OmEEK . For instance the value at 200 (approx 0.17%) is the probably that the drive fails during the year 201 for the first time. On a more useful scale you can see that the probability is almost constant during the first years (approx 0.35%): prnt.sc/WbTATvAyp3xo
    – PierU
    Commented Aug 30, 2022 at 11:46
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    More useful is the graph showing the probability that the drive lasts at least x years: prnt.sc/u6MXRgeYLrdH / prnt.sc/ulsjWmnNh3nC . You can see that there are 96% chances that it lasts at least 10 years.
    – PierU
    Commented Aug 30, 2022 at 11:50
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    @PierU I've got this interesting answer: Hello Jan, Thank you for your reply. I am sorry but variance of MTBF is not publicly available information. If you have any further questions, please reply to this email and we will be happy to assist you further. Sincerely, Angelo P. Western Digital Customer Service and Support westerndigital.com/support
    – user865044
    Commented Sep 1, 2022 at 8:16

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