The Case Of the Missing Megabytes
My computer is listed as having a 250 Gb hard disk, but when I check the available space, I only get about 225 Gb. What's wrong?
Nothing is wrong, exactly; you've just stumbled onto one of the facts of life about today's computers: The difference between hard disk size and usable space.
As a general rule, you can expect to lose about one-quarter of the space on a disk drive to things such as formatting and hidden partitions. You'll probably lose a smaller percentage on a smaller disk and more on a larger one. You'll lose toward the high side if the disk is the main disk on a system that came with the operating system installed and toward the low end if it's a second or third disk you've installed after you bought the computer. In other words, if you got 225 Gb out of a 250 Gb disk, you beat the guesstimate.
That lost capacity goes several places. One of them is the difference between formatted and unformatted size. Hard disk manufacturers quote the unformatted size of their products, but you have to format a disk to use it. This process of dividing it up a disk into blocks and setting up the tables and such to record and manage each block takes space.
Then there are the hidden partitions. On computers that ship with the operating system already installed, the manufacturers typically have already installed one or two partitions containing a complete system image, boot information and even parts of the Windows operating system. They do this in part because it makes computer troubleshooting easier and in part because it keeps you from using your recovery CD to copy Windows onto other computers in violation of your licensing agreement.
Should your primary (system) partition be destroyed due to a virus or malware, your computer vendor might opt to use the image on the hidden partition to restore the system to the way it was when you first got it. Users should have a backup plan in place where you regularly create an exact image of your hard disk. That way, should your disk fail, you can restore the latest image.
In cases where no backup image exists, some vendors provide a final, fall-back strategy. When you restore a base image from these hidden partitions, all the data, applications and configuration you added are lost. It cannot be overstates that using an image from a hidden partition should be a last resort for repairing a drive.
However, as mentioned, this is also a way of enforcing Microsoft's licenses. This might be convenient for Microsoft, but it is not beneficial for the user. Customers always should ask their systems vendors to provide them with a copy of the operating system on a CD, along with a valid registration key. That way, if you have to restore the operating system, you can do so.
Finally, in some cases, the capacity wasn't there to begin with. If you want to get technical about it, a KB should be 1024 bytes, not 1,000, because you're dealing with binary (base 2) notation. But when most people, including marketers, say "Kb," they mean 1,000 bytes. Similarly, a megabyte (MB) should be 1,024 KB, not 1,000, and so on. That terminological leakage adds up by the time you get into multiple gigabytes. However your computer reckons such things as disk capacity in binary and to a computer, a KB is 1,024 bytes. This is the standard practice in the industry and even though there has been a lawsuit filed challenging the "truth in advertising" over the 1,024 vs. 1,000 byte debate, it's not likely to change.
There is not a lot you, a user, can do about any of this capacity. The formatting loss in inevitable, the terminological difference is embedded in our system and messing with hidden partitions just to get more disk space is not a good idea. However, understanding the why your disk storage capacity isn't 100% what the advertising says it is should be a comfort.