I deliberately didn’t call it size because now we are looking at how much data the drive holds. This seems straight forward, but there is something to watch for.
Ever buy a new drive, get it home, and your operating system says it is smaller than it is supposed to be? The difference comes from us using a 10 digit number system, while the computer uses binary. If a drive says it has 100GB that means it has 100,000,000,000 bytes, in keeping with the conventions for prefixes indicating numbers (kilo, mega, giga, tera, and so on). The problem is that computers don’t consider a kilobyte to be 1000 bytes; their binary brains perceive a kilobyte as 1024, or 210. This is also true of megabytes, gigabytes and so on.
As a result your drive should have about (1000/1024)3, or 93% of the gigs listed on the box. A terabyte drive will have (1000/1024)4% of 1000, or about 910 gigs.
The International Electrotechnical Commission tried to clear this up in 1998 with the terms kibi, mebi, gibi, tebi and so on to indicate where the numbers are in line with the computer’s expectations; the **bi implies binary. It, sadly, hasn’t caught on.