The presentation of an HDD to its host is determined by its controller. Modern HDDs, such as SAS and SATA drives, appear at their interfaces as a contiguous set of logical blocks; typically 512 bytes long but the industry is in the process of changing to 4,096 byte logical blocks.
The process of initializing these logical blocks on the physical disk platters is called low level formatting which is usually performed at the factory and is not normally changed in the field.
High level formatting then writes the file system structures into selected logical blocks to make the remaining logical blocks available to the host OS and its applications. The operating system file system uses some of the disk space to organize files on the disk, recording their file names and the sequence of disk areas that represent the file. Examples of data structures stored on disk to retrieve files include the MS DOS file allocation table (FAT) and UNIX inodes, as well as other operating system data structures. As a consequence not all the space on a hard drive is available for user files.
NTFS (New Technology File System) is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7.
NTFS includes several new features over its predecessors:
NTFS Log: NTFS is a Journaling file system and uses the NTFS Log ($LogFile) to record metadata changes to the volume.
USN Journal: The USN Journal (Update Sequence Number Journal) is a system management feature that records changes to all files, streams and directories on the volume, as well as their various attributes and security settings.
File compression: NTFS can compress files using LZNT1 algorithm (a variant of the LZ77).
Encrypting File System (EFS): EFS provides strong and user-transparent encryption of any file or folder on an NTFS volume.
Quotas: Disk quotas allow Windows to set a threshold of disk space that users may use.