Apple Didn't Cheat on You (from tech_pickle)

I got a question yesterday from Scott about why his 20GB iPod only holds just over 18GB. Without research, I am going to try to answer this. When a hard drive is made, it is theoretically capable of holding (in close proximity to) the amount of data it is designed to capacitate. With every microprocess, however, there are bound to be errors (due to dust, material quality, processes used, etc.) in the product. When the iPod is turned on (or that hard drive is powered up and formatted), those bad spots, or bad sectors, appear and are marked as such in the drive's file allocation table (FAT). This is one reason the drive is below stated capacity.

Another reason is the file allocation table itself. The FAT (someone please let me know if I'm wrong) is the part of the hard drive that indexes every sector and keeps track of what files and directories are where on the drive. If a file is fragmented on the drive, every part of the file is kept track of in this table. This is how the computer knows how to access the file, even if it is broken up and spread about the disk. Am I getting away from the subject?

Anyway, the bigger the hard drive, the bigger this FAT has to be. That takes up a bit of space, too. My 40GB iPod actually says I have 37.1GB available. I imagine part of the space on the iPod is taken by the databases that keep track of all the song titles, photo properties, album artwork, and your notes/calendar/other stuff. This isn't accessible to the user, so it shouldn't be counted as useable space. Also, don't forget there's an operating system on the iPod itself. Mine probably takes up even more space since it's the iPod photo, which must provide tools to display the photos, and display color throughout the interface.

One more reason the capacity could be misstated: a difference in the measurement of the actual capacity. You see, a bit is the lowest unit of measurement for computer storage. A byte is made of eight bits. A kilobyte (KB), however, is not conventionally 1000 bytes; a kilobyte is actually 1024 bytes. Okay:

• byte: 8 bits


• kilobyte (KB): 1024 bytes


• megabyte (MB): 1024 KB


• gigabyte (GB): 1024 MB


• terabyte (TB): 1024 GB

I'd go over what 1024TB is, but the average consumer doesn't need to worry anyway for another ten years or so (I could be wrong about that), and this column isn't research-based.

Some manufacturers, for marketing purposes (so I've heard), measure the capacity with one kilobyte = 1000 bytes. Let's do the math: suppose you take a hard disk with a capacity of 40,000,000,000 bytes. Divide by 1,000 to get kilobytes, and you'll have 40,000,000 of them. Divide again by 1,000 and you'll have 40,000 megabytes. Now again for 40 gigabytes. There.

Now let's take the same drive and use the conventional measurement: 40,000,000,000 divided by 1024 to attain kilobytes: 39,062,500 kilobytes. Again, ~38,146 megabytes, and finally ~37.253 gigabytes. Sound familiar? Some unknown legal loophole lets them say 40 GB because that's the marketing department's hard drive measurement system, and it's easy to use. Every operating system I know of (including Windows) measures hard drive capacity in 1024-byte kilos.

Take a look here and see the small disclaimer in the footnotes: "1GB = 1 billion bytes; actual formatted capacity less."

So that's why your drive is almost never what it says. Want a way around it?

Buy two.