It doesn't apply to small read or writes. The harddrive has a physical arm that has to move back and forth in order to read and write on different areas on the drive, and it simply can't cope and deliver the "promised" 3Gb/s.
It's very much like having a car that can go 200mph, but if you have to turn all the time then you'd hardly reach top speed.
Sad, but that's how it is
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of cause, the amount of fragmented files would make it worse too. If you have a lesser fragmented partition, then the speed might be quicker, as the drive wouldn't have to work nearly as much - bottomline: defrag often
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