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Read vs. Write Performance
Hard disks can of course both read and write data (which isn't true of all storage devices.) The performance of a hard disk isn't exactly the same when it is doing a read as when it is doing a write, however. For some performance measurements there is no difference in how the system performs when doing a read or a write; for example, the platters are always spinning at the same speed, and so the latency of the drive doesn't change depending on what the heads are doing. Other measurements though, such as seek time, are different for reads as opposed to writes.
Almost all performance specifications given for hard disks are based upon how the hard disk performs while reading, not while writing. This is probably because hard disks spend more time reading than writing, and also because hard disks are generally faster when reading than when writing, so the numbers look better.
Some companies provide explicit write specifications in addition to their read specifications, while others do not. The most important specification that differs between reads and writes is seek time--a good rule of thumb is that the average seek time for writes on a hard disk is about 1 millisecond higher (slower) than the specification for reads. If a particular hard disk model doesn't mention the numbers you are interested in for writes, and if write performance is particularly important for your application, contact the manufacturer's technical support department. Someone there will know the answer, if you can get a hold of the right person. :^) It may be easier to try downloading the product manual for your model from the manufacturer's web site.