At the shop where I work, $HD is approximately equal to $GH + $OS and $OS is approximately equal to 2$GH
now i find this statement a little comfusing, because if i run the numbers, your saying that $GH is three times as %HD. so lets run the numbers and see if i am correct:

$HD = $GH + $OS
$OS = $HD - $GH

so therefore, $HD - $GH = 2$GH ----> $HD = 2$GH + $GH ----> $HD = 3$GH

okay, now that that has been sorted, i would just like to say that either, your HDD's are really cheap, which i doubt, or that customers are paying through the nose just to have their HDD's ghosted....

anyways, thats just my opinion.

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Thats a nice tut on how to avoid time wasting and spending too much. if the numbers you ran are standard it would also help people know (approxiamtly) how much its going to cost.