Mac Musings

Truth in Terabytes: Why Your Computer Says a 1TB Drive Is Missing 90 GB

Daniel Knight - 2007.08.10

Dr. Macenstein posted a provocative editorial late last week, Time for Hard Drive Manufacturers to Cut the GB BS. In it he bemoans the inflated capacities hard drive makers advertise compared to the number your computer reports.

Some of the details of his argument are wrong, but the gist of his piece is what makes it important. It's a call for consistency between the capacity a hard drive claims to have and the capacity our operating systems report.

Two Number Systems

We use two different kinds of numbers in the world of computers. We use the old fashioned decimal number system when we count and do spreadsheets, but when we're measuring memory or drive capacity, we use binary numbers that are roughly equivalent.

For instance, in the metric system, kilo means 1,000. In the world of computers, a kilobyte is 1,024 bytes. Mega, which is a million in the real world, is 1,048,576 in the binary world. Giga, a billion*, becomes gigabyte - 1,073,741,824 bytes. And the new number, a terabyte, isn't a trillion bytes; it's a bit more than 1,099,511,000,000 bytes (AppleWorks truncated the last two digits).

And when we reach a petabyte, instead of a quadrillion bytes, it will be 1.1259 quadrillion bytes.

When a hard drive has a 10 million bytes and is advertised as 10 MB, our operating system reports 9.5 MB. 10 billion bytes is called 10 GB by the manufacturer, but our computer tells us it's only 9.3 GB. And today's 1 TB drives only have 0.91 TB of storage space.

Why do drive makers do this? Because it sounds better to offer a 400 GB hard drive than a 372 GB one. And because they've always done it. If one company changes, it puts them at a marketing disadvantage.

Hard Drive History

Yes, it's been going on since the first 5 MB hard drives were attached to personal computers. And it's been confusing and frustrating users since the start of then.

It didn't seem so bad when a 20 MB drive showed up as 19 MB or a 40 MB drive as 38. But people began to notice and complain when their 80 MB drives were reported as 76.3 MB by their computers.

It got worse in the gigabyte era. An early iMac might have a 6 GB drive, but the Mac OS would report it as having 5.72 GB. The 80 GB drives of a few years ago were only 74.5 GB according to the operating system.

As we move into the terabyte era, the difference between advertised capacity and capacity as reported by computer operating systems will grow to 9%.

What Is Truth?

The truth is that both sides are right. The manufacturers are providing exactly the number of bytes they claim - using decimal numbers. And the operating systems are reporting exactly the number of bytes they see - using binary versions of kilo, mega, giga, and tera.

One proposal, which has not flown, is to add an "i" in the middle of binary measures, so what we call a MB today would become a MiB, a GB would become a GiB, and a terabyte at TiB. The theory is that this would eliminate the confusion between advertised capacity and what the operating system reports.

The problem is that operating systems don't work that way, and people are used to the current messy situation.

Time for a Class Action Suit?

There is precedence for bringing a class action suit against an entire industry for misleading practices. Perhaps the biggest example took on the entire field of CRT computer monitors. As with television sets, monitors were advertised by the diagonal size of the screen (which sound much more impressive than the horizontal dimension). Problem was, they were measuring the size of the picture tube itself, not the portion of the screen visible to the viewer.

Eventually someone filed a class action suit against pretty much everyone, and the final settlement required the entire industry to stop advertising tube size and instead advertise the diagonal of the viewable area.

As with hard drives, it would have been a disadvantage for Apple or Sony or anyone else to use the real world measurement when the rest of the industry continued to advertise tube size. The same holds for hard drives.

The only way to change this is to force the entire industry to move to reporting MB, GB, and TB the same way all of our computer operating systems do - and maybe give manufacturers enough leeway to round off numbers a bit, so today's 80 GB hard drive, which measures 74.5 GB, could be called a 75 GB drive. We could leave it up to the industry or the lawyers to determine how much of a fudge factor would be allowed.

This would once and for all eliminate the difference between digital and binary numbers as used for hard drives, flash memory, etc. And it would let the few holdouts who insist on reporting MiB and GiB drop the confusing new terminology in favor of what everyone (outside of drive makers and a few anal geeks) already uses.

So, who's going to step up to the plate and force the drive makers to use the numbers our computers use?

* We're writing for the North American audience, where a billion is a thousand million and a trillion is a million million. In other parts of the world, a billion is a million million, and so on. It gets messy....