The most common way to slow down any hard drive is to have too many individual streams of data too and/or from it at the same time.
I find and can prove, that copying from one drive to itself is exactly half as fast as from one drive to another, depending on the hardware obviously, equal drives and such.
This is a huge timesaver if you are extracting a lot of data from one drive to another, basically never extract from one drive to itself, it's twice as fast or more to extract archives to a different drive to save half the time.
Some of the newsgroup downloading programs like "grab it" have a 'folders' section where you can specify to download to one folder on one drive, and then extract to a different drive/folder, I find this saves a lot of extract time.
HDTune
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