In terms of the "stated" capacity of a drive, manufacturers always pretty much state that in terms of millions or billions of bytes "decimal", as has already been said here. And if a drive is advertised as (say) 120GB it will in fact have an "unformatted" capacity of "at least" 120,000,000,000 bytes. And if internally all drives were created eqally then they would all have the same "formatted" capacity, the same "usable" capacity, and they all would have the same capacity when you stated it in terms of a megabyte equalling 1,048,576 bytes. But all drives are not created equal.
The drives of different manufactures, even drives within different lines of the same manufacturer, can and do have diffrences in terms of things like the number of platters, ariel density, etc, etc. And its because of these differences that, say, a 120gb drive from Maxtor ends up showing more usuable space than one from Western digital. If any two drives of the same "true" size, regardless of manufacturer, are formatted they will always yeild identical capacities. You don't loose more with WDC than Maxtor (say). The reason for any difference is strictly due to how many actual sectors (logical blocks) are on a drive in the first place.
I said above, a drive advertised as 120gb will always have 120gb (decimal) of capacity. But, it may have more (and always has a bit more). Its this excess capacity originally in the drives that makes for the differences.
Generally this excess is not advertised, rather the drives capacity is stated as a rounded down (never up) number. So a drive that in fact may have 122gb of capacity will just be advertised/sold as a 120gb. Likewise, a drive with 120.5gb capacity will also be sold as a 120gb. In both cases the consumer is getting (at least) a 120gb (decimal) drive, in fact they are getting more than a 120gb drive. Its just that in one case vs the other the "more" is larger. As I said, generally there is no stating/advertising outright of this excess. When drives were smaller you use to see "odd"/fractional sizes mentioned more, but today...
Maxtors traditionally have more usable sectors in their drives of a given size than, say, Seagate or WDC (Segate and WDC tend to be pretty much the same). So for example... A maxtor DiamondMax Plus 9 120gb drive has 240,121,728 usuable sectors (at 512 bytes per sector). That means it has an unformatted capacity of 122,942,324,736 bytes. A seagate Baracuda 7200.7 Plus 120gb drive has 234,441,648 usable sectors, yielding a capacity of 120,034,123,776 bytes. Finally, the WDC Cavier WD1200JB SE 120gb has 234,441,648 usable sectors , yielding a capacity of 120,034,123,776 bytes. All three drives have a capacity of at least 120gb (decimal), or more. The Seagate and WDC have the same capacity, but the Maxtor has more. Now it doesn't matter whether you are talking capacity in terms of formatted or unformatted, decimal or not, etc, the relative capacities will remain the same. You don't loose more with one brand vs the other. You simply start out with more (in the case of Maxtor).
So things may be a bit confusing, but when a consumer buys a xzy size drive they are getting at least that size drive no matter what (in decimal terms anyway). And since it is only being advertised as xyz in size one cannot really complain if they get more than that. It may be better/reasonable though to state things in terms of "formatted" capacity perhaps, but...
