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                                                                                                                  Thermal Reliability
                                                                                                Cool-Running WD Hard Drives Demonstrate
                                                                                    Exceptional Reliability in High Duty Cycle Environments




Overview/Executive Summary
WD drives are highly reliable because they are cool-running and thus perform exceptionally at high temperatures and
high duty cycles. The thermal reliability of WD drives can be quantified by tests that prove this assertion.

Test i n g C a t e g o r ies
WD performs rigorous testing to generate data that quantify drive reliability at high temperatures and high duty cycles. The tests are
divided into these categories:

  !   Reliability demonstration test (RDT) -- This is a one-time test at the qualification stage, typically with 1000 drives running for
      1000 hours, to confirm that a hard drive design meets customer requirements for reliability before going into volume production.
  !   Ongoing reliability tests (ORT) --These tests are performed weekly during a production run. A sample comprising a certain
      number of drives is drawn weekly and tested over a four-week interval to ensure that the drives meet reliability requirements.

Customer Requirements and Testing
In regard to testing, WD customers want to know the details of how we arrive at the numbers we cite in our literature and how these
values may affect their operating environments. WD test conditions typically involve a high temperature and a 100% duty cycle.

The duty cycle is the percentage of time that a drive is reading, writing, or seeking. So, if a drive is engaged in these activities 12 hours a
day, or half the time, then its duty cycle is 50%. The percentage of time a hard drive is used functionally is the functional duty cycle; the
percentage of time a hard drive is powered on is the power-on duty cycle. WD typically assumes a power-on duty cycle of 3120 hours-
per-year (35.6%) for desktop drives and 8760 hours-per-year (100%) for our enterprise drives. In the latter case, 100% means that the
drive is powered on all the time.

One way that WD customers measure drive reliability is the mean time to failure (MTTF) value. This is the average time (expressed in
hours) that a component works without failure. It is calculated by dividing the total number of operating hours observed by the total
number of failures.




                                                                     Develop formulas                    Use the information
                                                                    to determine MTTF                    to predict MTTF at a
                                  Perform reliability                 at temperatures                         customer's
                                      life tests                      and duty cycles                         anticipated
                                                                     of interest to out                        operating
                                                                         customers                         temperature and
                                                                                                               duty cycle

                                        Step 1                             Step 2                              Step 3



                                                        Figure 1. Reliability Prediction Process Steps




                                                                            -1-
Figure 1 is an overview of the steps in reliability testing. It begins by performing reliability life tests as noted in step 1.
Then, in step 2, our advanced reliability engineering department models the results in equations. Finally, in step 3, these
equations provide a prediction of reliability for a given customer at any operating temperature and at any given value of
functional duty cycle.

WD tests and qualifies its drives under precise conditions relating to temperature and duty cycle. Once we have
knowledge of how long our products last, we then are able to transform our test results to the operating conditions that
our customers plan for their products that use our drives. Almost without exception, customers operate drives at different
temperatures and duty cycles than we employ in our tests. For instance, if we test drives at a temperature of 55

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