When is a Gb not a Gb and why doesn’t it matter?

Let’s suppose that you’ve finally decided to buy an external drive to back up at least some of your important files. Maybe you’re doing this just because you know it’s a good idea or maybe you are a computer support client of mine and you’re tired of me dropping very heavy hints about backups.

Windows Explorer Drive Capacity
How Windows Displays the Drive Capacity

I’m going to use a 160gb drive as my example for the rest of this blog. So, you connect the new drive to your computer, you open Windows Explorer to see what it makes of it and, lo and behold, it looks as if you’ve been sold a pup. Windows tells you that this is not a 160gb drive but a 149gb drive and although it’s brand new there’s less space available than the size of the drive. The difference between the 149gb (the capacity reported by Windows) and the 160gb (the capacity that the nice people in PC World sold you) is almost 7% of the total and that’s enough for a grumpy old man like me to feel cheated.

Actually, there are a number of things going on here. The most important is that the drive manufacturer and Windows are not agreeing on what constitutes a “gb” (gigabyte).

These days, 1gb = 1000,000,000 bytes (or 1000 megabytes). This is the designation that drive manufacturers use when labelling and selling their products. So, a 160gb hard drive means 160,000,000,000 bytes.

1gb used to be thought of as something different. This was derived from the fact that 1kb was not 1000 bytes but 2x2x2x2x2x2x2x2x2x2 bytes – ie 1024bytes. Calculating it this way, 1gb is actually 1,073,741,824 bytes. Strictly speaking, this unit is now called a gibibyte (GiB) but I don’t think I’ve ever heard anyone use that word.

So, I can multiply the 149gb that Windows thinks it has  X 1.073741824 bytes and get back to the 160gb which PC World claimed to have sold me.

Notice that Windows is reporting only 148gb of the 149gb as being available. There’s nothing I can do about that. It’s just pinched some space for its own use.

Seagate GoFlex 2.5 inch Drive
A Seagate GoFlex 2.5 inch Drive

Actually, your own brand new drive may be showing less space available for another reason. The drive may include free backup software. Defintely worth looking out for if you want a reasonably straightforward backup solution.

Does all this talk of space available on your new drive actually matter? Not really – unless you are paranoid or grumpy and think that the drive manufacturers are trying to cheat you.

So why am I telling you this? Mainly because you may have noticed the anomaly and wondered what was going on. I’m also taking advantage of the opportunity to emphasize that one of the best bits of computer advice I can give is that it’s a very good idea to have backups of at least your most important data files. Over the 27 years I have been providing computer support I have seen several instances of people losing their data because of something like a hard drive failure or virus attack. It can be very upsetting, and potentially serious if your livelihood is involved.

Another reason why we needn’t try to be too pedantic in working out how much space we’ve got available on the new drive is that we always need to leave a fair chunk of it unused. We need to allow a minimum free space of about 15% “wiggle room” if we are going to be regularly reading and re-writing the files on the drive. If any drive gets filled beyond about 85% capacity then the performance starts to degrade as the operating system struggles to effficiently store the data and read/write it from/to the drive. So, it’s always best to buy a drive that’s larger than your apparent needs.

Finally, the main reason that it doesn’t really matter too much about accurately measuring capacity against your precise needs is that the capacity of external drives is going up much faster than our need for all that space. It’s hard to buy a drive of smaller than 340gb nowadays and the best value in terms of “£ per gb” is probably a 500gb drive. The only thing that’s likely to use up that kind of capacity is a large movie collection. A large photo collection and a large music collection added together with all of your normal data files are unlikely to be a problem.

I know I’m always banging on about backups but there’s another reason why it may be a good time to act – external hard drive prices (in fact, all hard drive prices) may go up in the coming months. Floods in Thailand have wiped out a significant percentage of the manufacturing capacity and this could have a serious impact on prices.