Why does a drive never give you the full capacity?

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Some people, the more inquisitive, may have noticed that their new 8GB USB Flash drive for example doesn’t quite provide them with 8GB of storage capacity.
This may lead to doubts about the drive being genuine or mislabelled, but if the discrepancy is small then there is no need to worry.
The reason for this is because of the way computers manage disk drives. In the very old days when computers were operated by men in white coats in laboratories. When they finished running their program they would save their data to disk, which isn’t unusual.
But what was unusual was the process of saving, this would involve issuing commands to the computer to start the disk motor, move the disk head to an empty position on the disk, commencing the data transfer to the disk etc etc.
Someone had the idea of making a small program which would look after the storage and retrieval of data from a disk, combining all the complicated and tedious commands with just one command, e.g. “Save”.
The Disk Operating System was created. Commands like Load or Save would start an automated process which would just deal with the task of saving and retrieving files from a disk, this was much less work for the computer scientists.
What was clever about it was that the system would keep track of where it wrote files to. It would create a File Allocation Table on the disk and note within this, which sector and clusters on the disk a particular file was saved. When a user requests to load a file, the file name is looked up in the File Allocation Table and the actual file can be loaded from the relevant section of the disk. It is this which takes up space on a disk.
Imagine having a new blank note book. This notebook would have a capacity of so many letters for example. If you then wrote “Table of Contents” on the first page, followed by “Page 1, Page 2, Page 3 etc”. Then on each page, you wrote “Page 1 , Page 2 etc” on the corners, you would have reduced the amount of letters you can write in the book (because you’ve just written some in).
You may still regard the book as empty, as there is no useful information in it, but to a computer it is not empty. It has characters like “Page 1” written within it. This is the process of formatting a disk drive. Formatting a disk involves the Filing System writing data to the disk, thus reducing its capacity, even though a user has not saved any files to the disk yet.
So why did the disk drive manufacturers decide to quote unformatted capacity, and not formatted capacity? The reason being is that at the time there were so many different operating systems which would use a different filing system, which would format the same disk in a different way, thus resulting in a different formatted capacity. The manufacturer would not know if the disk they sold would be used in a DEC VAX using VMS or a Window system using NTFS. Therefore disks have always been sold stating their unformatted capacity.
It is not possible to use a disk unformatted within a computer system, therefore you will never be able to buy a disk and have its stated capacity available to you for your purpose.
This applies to any kind of drive, it does not have to be a disk drive, but can be a USB Flash Drive, Compact Flash Card or Memory Stick for example. Because there is a different array of operating systems in use today (Windows, MAC, Linux etc) each using their own filing system, the industry has settled on a standard filing system for such devices like USB Flash Drives and Memory Cards, it is called FAT32.
FAT (File Allocation Table) was introduce in Windows and has been improved over the years to allow for larger drives, to its current FAT32 revision. Windows has now moved over to its secure NTFS filing system which was introduced with Windows NT. FAT32 was last seen on a PC in Windows 98SE / ME. However, it is ideal for removable drives like USB Flash drives, and provides cross compatibility with other systems like Macs. As Windows can use FAT32 even though its native format is now NTFS, so can the Apple MAC, and even Linux systems like UBUNTU.
Because FAT32 is now a universal standard, Flash memory now comes pre-formatted so that users do not need to format them, like they would a new hard disk.
Therefore, remember, even though you haven’t formatted your new Flash Drive or Memory Card, it is formatted, and hence its capacity will be less then its stated capacity.
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