A hard drive is formatted to give it a logical structure that a computer and its operating system will understand. Three operations are generally classified as formatting: low-level formatting, partitioning and high-level formatting. Each of these operations is necessary before the applications that run on a computer can use a hard disk.
The low-level formatting process creates the basic logical structure of a hard disk. When disks are produced they are completely blank; low-level formatting gives structure to a disk so that disk controllers can read and write to it properly. A low-level format will divide a disk into tracks and sectors and add information so that disk controllers can locate those tracks and sectors quickly. Low-level formatting completely destroys all data on your hard disk, removing the operating system, applications and any files you had saved. Most hard disks now come with low-level formatting already applied.
Partitioning divides a single physical disk into one or more logical drives. In a Microsoft Windows system, for example, you might partition a disk into a “C” drive and a “D” drive. Even though you only have one physical hard disk in your computer, the Windows operating system treats it as if there were two disks. You can also partition a disk so that one logical drive is used by one operating system and another logical drive is used by another completely different operating system. This is a multi-boot environment where you could choose to boot your computer into Windows or Linux.
High-level formatting is the process of creating a file system on the disk so that the applications you run on your computer can read from and write to files. File systems are usually operating system-specific and examples of file systems are: FAT and NTFS, used by Windows, and EXT2, EXT3 and EXT4, used by Linux or Unix. Whereas low-level formats operate on the entire disk, high-level formats operate only on logical partitions, so you can high-level format one partition without affecting any other partitions.
High-level formatting a drive overlays a new file system on the old one without removing the old data. Old data is only overwritten when the new file system writes something to a part of the drive that was used by the old file system. With specialist tools, it is still possible to access old data that has not been overwritten. High-level formatting is therefore insecure and if you wish to ensure confidential information is properly overwritten, use either a low-level format or an application that securely overwrites deleted data. Note that high-level formatting a drive will make the old data inaccessible without those specialist tools, so think carefully before performing any sort of format.