I think we need to know a little more on the context of that question. Full capacity? Model capacity? There's probably many ways you can define that depending on context.
Maybe this short explanation will answer your question though:
You buy a "40GB" drive. Supposedly 40GB. Drive mfg's count 1GB = 1 billion bytes, so the drive has 40billion bytes of storage (usually a little more but close enough for our purposes). Then the drive is formatted and some of the space is used in the boot area, directory area, file allocation tables, and formatting information between each sector of the disk. This makes the 40GB drive have roughly 39G left (+/- a little depending on many factors).
Then, when the OS (Windows, Linux, whatever) gets it, it calculates 1GB as 1024MB, and 1MB is 1024KB, and 1KB is 1024bytes (making 1GB = 1,073,741,824bytes) so it divides 39billion bytes (approx) into that to get only 36GB give or take.
Thank you! I got the terms from the spec of a Hitachi disk drive, HDS722516VLAT20. The full capacity is 250GB and the model is 160GB. The concept you explain may be more like the differnece we see between the disk size and the size on file manager in Windows. However, for the model capacity, it is supposed to be a spec for unformatted hard drive, I guess. Do you think so?
No, what you're refering to is simply a matter of how many platters are used and the size of the platters. For the 7K250 series, there are a maximum of 3 80GB platters. 8x3=24, so if that 80GB figure just happens to be "a hair larger than 80GB" they can round it up to 250GB for all 3.
The 160GB drive only has 2 of those platters. The 120GB also has 2 80GB platters, but with one side not used (subtract half the capacity of the platter). The 200GB drive has 3 platters with 1 side of one platter unused.
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