Disk formatting
[6] Special tools can remove user data by a single overwrite of all files and free space.
The disk subsystems and other direct access storage devices on the IBM System/360 expanded this concept in the form of Count Key Data (CKD) and later Extended Count Key Data (ECKD); however the use of variable block size in HDDs fell out of use in the 1990s; one of the last HDDs to support variable block size was the IBM 3390 Model 9, announced May 1993.
Different low-level formats can be used on the same media; for example, large records can be used to cut down on inter-record gap size.
[14] After establishing the structure of tracks, a formatter also needs to fill the entire floppy and look for bad sectors.
Traditionally, the physical sectors were initialized with a fill value of 0xF6 as per the INT 1Eh's Disk Parameter Table (DPT) during format on IBM compatible machines.
With the media, the drive and/or the controller possibly procured from separate vendors, users were often able to perform low-level formatting.
[g] User-instigated low-level formatting (LLF) of hard disk drives was common for minicomputer and personal computer systems until the 1990s.
Typically this involved subdividing each track on the disk into one or more blocks which would contain the user data and associated control information.
Modern hard drives can no longer perform post-production LLF, i.e. to re-establish the basic layout of "tracks" and "blocks" on the recording surface.
Reinitialization refers to processes that return a disk to a factory-like configuration: no data, no partitioning, all blocks available to use.
[20] Although the SCSI command provides many options, even resizing, it does not touch on the track layer where low-level format happens.
[25] When the hard drive's built-in reinitialization function (see above) is unavailable due to driver or system limitations, it is possible to fill the entire disk instead.
On newer drives with defect management, reallocated sectors may be left unerased, whereas the built-in re-initialization function will erase them.
Doing so voids the plausible deniability of the process, as the encrypted areas (indistinguishable from random without a key, unless the cipher is compromised) will stand out among zero blocks.
Since much of the low-level formatting process can today only be performed at the factory, various drive manufacturers describe reinitialization software as LLF utilities on their web sites.
On MS-DOS, Microsoft Windows, and UNIX-based operating systems (such as BSD, Linux and macOS) this is normally done with a partition editor, such as fdisk, GNU Parted, or Disk Utility.
Partition editors and ICKDSF today do not handle low-level functions for HDDs and optical disc drives such as writing timing marks, and they cannot reinitialize a modern disk that has been degaussed or otherwise lost the factory formatting.
IBM operating systems derived from CP-67, e.g., z/VM, maintain partitioning information for minidisks externally to the drive.
High-level formatting is the process of setting up an empty file system on a disk partition or a logical volume and for PCs, installing a boot sector.
Formatting an entire logical drive or partition may optionally scan for defects, which may take considerable time.
In current IBM mainframe operating systems derived from OS/360 and DOS/360, such as z/OS and z/VSE, formatting of drives is done by the INIT command of the ICKDSF utility.
Reformatting is a high-level formatting performed on a functioning disk drive to free the medium of its contents.
In such cases, the user's data remain ripe for recovery with specialist tools such as EnCase or disk editors.
Reliance upon /U for secure overwriting of hard drive partitions is therefore inadvisable, and purpose-built tools such as DBAN should be considered instead.
On Linux (and potentially other systems as well) mkfs is typically a wrapper around filesystem-specific commands which have the name mkfs.fsname, where fsname is the name of the filesystem with which to format the disk.
Examples include GNU Parted (and its various GUI frontends such as GParted and the KDE Partition Manager) and the Disk Utility application on Mac OS X.
