Hue Jass
I have spent about an hour figuring this out (it was easy, i was just making it harder that it should have bee). I have found out the specific percentage of usable harddrive space on an average harddrive.

On a harddrive, you can only use a certain amount of that harddrive. First of all, i would like to know 'exactly' what that space is for, i have an idea that it's for the FAT/MBR (depending on which os you use) and the firm ware, or eaither or. here is an example:
40GB harddrive - 37.25GB usable space
my 160GB raid array - 149 GB usable space
the formula to figure out how much space you can obtain:

total drive space X .93125 = usable drive space

Now about my first question, don't you dare say that, that space is taken up by the operating system, all of my friends who think they are god of computer knowledge say that and it's not. I can even prove this to you: take a harddrive and format, but don't put a single thing on it (set as slave so you can see it) it will have that certain ammount of unusable space on it.
hope that helped some of you guys
It has to do with what Microsoft cosiders 1gb and what Harddrive makers consider 1gb, they have confilicting amounts of MB/KB/etc.
Hue Jass
but i have seen it on other operating systems, not just os' from MS
your equation worked with my western digital 20gb running windows, 18.6gb.
my 40Gb one reads 38.2Gb in windows

41.1Gb in Speedfan