XP Showing HardDrive Size Smaller Than Physical Size - Why?

Hi all -
Scenario is this - XP Pro XP2 w/ updates. Three Western Digital harddrives - 1 x 120g, 2 x 320g.
XP shows the 120 as 111.79 and the 320's are shown as 298.09 ....
I've just bought the 2nd 320 as a recovery disk to image the first 320 to - and it tells me it's too small!
So .... XP is telling me 298.09 .... does it just 'report' it that way for some damn reason OR are my 320's acutally physically smaller than what's on the label and if so does anyone know why?

(Accept my appologies in advance if this type of question has been covered before but I could not find a similar question in my searches.)

Thanks very much in advance .....

(anyone want a week old 320? Grrrrrrrr!!)

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ibu1Connect With a Mentor Commented:
here's the explanation:

Most operating systems define a hard disk drives capacity using binary or base-2 mathematics. This translates to 1 gigabyte (GB) equal to 1,073,741,824 bytes. This is the correct value when using binary or base-2 mathematics.

However, hard disk drive manufacturers define drive sizes using base-10 mathematics, in which 1 GB is equal to 1,000,000,000 bytes (rather than the 1,073,741,824 bytes, as listed above).

This discrepancy in reporting drive sizes (base-2 vs. base-10) may lead you to believe that you have a hard disk drive of less than expected capacity if you compare the figure reported by the operating system with the figure reported by your documentation, although the actual hard drive size is identical. Microsoft® Windows® simply counts the size differently, and will report a different, slightly smaller, figure.

so basically the hard drive manufacturers and the OS programmers used different bases that led to the discrepancy.
hope this helps!!
KCTSConnect With a Mentor Commented:
... and in addition to ibu1's very accurate explanation of how the binary calculations lead to dicrepancies, there is the additional factor that manufacturers also quote the 'non-formatted' capacity. Once the disk is actually formatted some capacity if lost in the process as it it used to hond disk info, directory structure etc.. This and the binary maths issue resuts in typcal sizes being in the region of 10% lower than the manufacture's published capacity.
DAQLDAuthor Commented:
Gents, thank you both.
My lamen response? That sucks.
Especially the "This and the binary maths issue resuts in typcal sizes being in the region of 10% lower than the manufacture's published capacity" part ... makes you wonder what other things we don't know.
If I find out tomorrow that my new C2D is only as good as first Celeron 400, I'll peak! :0

I'm at a bit of a loss as to my my '320' image won't fit on my empty '320' disk but I guess that a whole other set of points .... I guess I can look forward to spending another $200+ tomorrow on a 500Gig that's only going to be around 460 by the sounds of things.
Thanks again for your explanations and solving this little mystery for me - I'll split between you both.
DAQLDAuthor Commented:
Just as a footnote - my 400 gig drive clocks in at 372.61gig.
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