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.
Next: Component vs. System Performance
