As a public service I am re-posting this which is from my original blog. Toward the end of my other blog this became one of the more popular searches to my site. It really upset me how hard drive companies could be so devious so hopefully this will help explain some of what is going on.
Now get your mind out of the gutter and listen to what I have to say. For those who read my blog on a regular basis know, I recently bought a new HD, a Western Digital 120GB . I'm very happy with it in terms of how much more space I have now for all my stuff but it really does bother me that they claim their hard drive is 120 GB because it is not.
Anyone who knows something about programming knows that a mb is not 1,000,000 bytes even though mega means "1,000,000". In computer terms a megabyte is actually 2^20 bytes or 1,048,576 bytes. Equivilently, a gigabyte is not 1,000 megabytes, it is 2^10 megabytes or 1024 megabytes. Doing the math you can see that 120 GB really should be 122,880 megabytes. But that's not how Hard Drive manufacturers do it. They take the traditional approach of Mega meaning 1,000,000 and Giga meaning 1,000,000,000. Therefore when they claim 120 GB they really mean 120,000,000,000 bytes. Doing the conversion you will find that this means there are 114,440 megabytes or 111.8 GB. Quite a difference.
Why do they do this? Well like I said above SIZE MATTERS!. They are technically not lying to the consumers when they tell you that their drive is 120GB because in a traditional sense they are correct. In computer terms they are dead wrong and this is why when you install a hard drive you will always get significantly less room then you think you will. However, a hard drive manufacturer has no incentive to be honest with you because everyone else is not. Therefore if he advertised a Hard Drive as 111.8 GB (the correct number) instead of 120 GB (the wrong number) the casual consumer would think that the 120GB was more than 8 GB bigger even though they are exactly the same.