Why are Hard Drives listed as 3.5 inches?

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I was just looking on Ebay for a hard drive and they list them as 3.5
inches, when in fact they measure 4 inches. (I just measured one of
mine). Just curious!!!

I'm referring to standard desktop IDE or SATA drives, not the smaller
ones for laptops.

[VanguardLH[_2_]]

wrote:

> I was just looking on Ebay for a hard drive and they list them as 3.5
> inches, when in fact they measure 4 inches. (I just measured one of
> mine). Just curious!!!
>
> I'm referring to standard desktop IDE or SATA drives, not the smaller
> ones for laptops.

Example of specifications for WDC HDD:
http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-771434.pdf

Notice the width is listed as 4 inches. That's because the case has to
be larger than the platter(s). See:

http://www.datarecoverytools.co.uk/wp-content/uploads/2009/08/form-factor-reviews-1.jpg

3.5" refers to the drive's "form factor", not its actual physical
dimensions.