Seagate doesn't like the words decimal and binary, especially in the same sentence. The definition of each these words is costing the hard-drive manufacturer some serious money, and it's all to do with quoted hard-drive capacities (decimal) versus operating systems' interpretation of file sizes (binary).

Trouble is, hard-drive manufacturers quote drive capacity in the decimal (base 10) format. Now, one kilobyte, when evaluated in decimal, intimates 1,000 bytes (10 to the power 3) and one gigabyte intimates 1,000,000,000 bytes, and there's nothing logically incorrect about that.

However, operating systems have historically calculated capacity in binary terms, so base 2, derived from the method by which computing memory is addressed. Taking this line of thought and extrapolating outwards, one 'kilobyte' as your operating system sees it, would constitute 1,024 bytes (2 to the power 10), and one 'gigabyte' 1,073,741,824 bytes (2 to the power 30). It's clear to see that as the capacities increase in orders of magnitude, the difference between decimal and binary interpretations grow exponentially larger: a binary 'kilobyte' is 2.4 per cent larger than a decimal, and a binary gigabyte is 7.3 per cent larger.

The difference between decimal (hard-drive capacities) and binary (operating system file-size interpretation) is one that's been laboured because it's pushed Seagate into deep water, charged with misleading customers, but it's not as clear as that, really.

Let's take a 500GB hard drive as an example. Seagate, or any other manufacturer for that matter, claims its capacity based on the decimal system, so 500,000,000,000 bytes. There is nothing factually incorrect with expressing the capacity in the manner, when evaluated in isolation. However, the increasing divergence between decimal and binary means that good old Microsoft Windows Vista, calculating file sizes in binary, reckons that your shiny hard drive contains a mere 465.66GB. Customers look at the Microsoft-quoted figure and exclaim, 'Seagate, dude, where's my 34.34GB gone?' It's gone nowhere because it never existed in the first place - the difference was wholly a matter of how you interpret the size. Inherently hard-drive capacity can be quoted in either decimal or binary; it has no base system of its own, so hard-drive manufacturers prefer to display the decimal gigabyte, which makes for a larger and more-impressive number.

Indeed, such has the consternation been between what constitutes a GB, the binary equivalent (which is always larger, exponentially so as the capacity rises. labouring the point some more) is correctly referred to as a GiB (gibibyte) and not GB, and is naturally at loggerheads with the decimal gigabyte. 500GB is the same as 465.66GiB, then, but confusion reigns when Windows outputs in GB but actually means GiB.

The thorny issue of definition and using the 'biggest number' has, according to an article in Computerworld, led Seagate to admit that, in gigabyte/gibibyte terms, its drives contained around 7 per cent less capacity than the operating system saw - as calculated by the OS' binary interpretation, remember. US-only customers who bought drives between March 22, 2001 and December 31, 2006 will be eligible for a five per cent cash refund of the drive's net price. Customers who bought drives after the cut-off period will be entitled to free backup and recovery software.

Seagate has been punished, somewhat unfairly from our point of view, for not adhering to the binary interpretation of capacity. The matter should have been cleared up over 20 years ago by either hard-drive manufacturers adopting the binary system of reporting or, as is now the case, new suffixes for binary capacities - KiB, MiB, GiB, etc. A GB is not the same as a GiB and Seagate is the first to pay for it.