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Disk formatting is the process of preparing a hard disk drive or flexible disk medium for data storage. In some cases, the formatting operation may also create one or more new file systems. The formatting process that performs basic medium preparation is often referred to as "low-level formatting." The term "high-level formatting" most often refers to the process of generating a new file system. In certain operation systems (e.g., Microsoft Windows), the two processes are combined and the term "format" is understood to mean an operation in which a new disk medium is fully prepared to store files. Illustrated to the right are the prompts and diagnostics printed by MS-DOS's FORMAT.COM utility as a hard drive is being formatted.
Formatting a disk for use by an operating system and its applications involves three different steps.
Low-level formatting (i.e., closest to the hardware) marks the surfaces of the disks with markers indicating the start of a recording block (typically today called sector markers) and other information like block CRC to be used later, in normal operations, by the disk controller to read or write data. This is intended to be the permanent foundation of the disk, and is often completed at the factory.
Partitioning creates data structures needed by the operating system. This level of formatting often includes checking for defective tracks or defective sectors.
High-level formatting creates the file system format within the structure of the intermediate-level formatting. This formatting includes the data structures used by the OS to identify the logical drive or partition's contents). This may occur during operating system installation, or when adding a new disk. Disk and distributed file system may specify an optional boot block, and/or various volume and directory information for the operating system.
Partitioning is the process of writing information into blocks of a storage device or medium that allows access by an operating system. Some operating systems allow the device (or its medium) to appear as multiple devices; i.e. partitioned into multiple devices.
On MS-DOS, Windows, and UNIX-based operating systems (such as BSD, Linux/GNU, OS X) this is normally done with a partition editor, such as fdisk, parted, and Disk Utility. These operating systems support multiple partitions.
In current IBM mainframe OSs derived from OS/360 and DOS/360, such as z/OS and z/VSE, this is done by the INIT command of the ICKDSF utility. These OSs support only a single partition per device, called a volume. The ICKDSF functions include creating a volume label and writing a Record 0 on every track.
Floppy disks are not partitioned; however depending upon the OS they may require volume information in order to be accessed by the OS.
Partition editors and ICKDSF today do not handle low level functions for HDDs and optical disk drives such as writing timing marks, and they cannot reinitialize a modern disk that has been degaussed or otherwise lost the factory formatting.