Hard Drive Structure: Tracks, Sectors, and Logical Organization

Each face of the plate or disk surface is divided into a number of concentric rings, called tracks. The number of these tracks depends on the sensitivity of the mechanical parts inside the disk.

Each track is numbered from zero, increasing as we move towards the center of the disc.

Speaking of tracks also means talking about cylinders. The number of tracks per surface equals the number of cylinders. If the positioning arm is on a certain track, it is on the same track on all disks, and the set of all those tracks is called a cylinder.

The tracks are divided into sectors, which hold the same amount of data (512 bytes). However, these tracks do not always have the same number of sectors. This depends on the length of the track. For example, the outer tracks are longer and have more sectors compared to the inner tracks, which have fewer sectors.

Zone Bit Recording (ZBR) involves placing a different number of sectors per track on a surface.

These sectors are grouped into clusters or allocation units. They are the smallest unit of information that a hard drive can read or write, and are numbered from 1.

Logical Structure of a Disk

MS-DOS stores information according to the disk's structure. The logical structure is achieved when the disk is formatted and consists of:

• Boot Sector
• File Allocation Table
• Root Directory
• Data Zone

The Boot Sector

The boot sector is the first sector of any hard drive (head 0, cylinder 0, sector 1). It stores the partition table and a small master boot program, also called Master Boot Record. This program is responsible for reading the partition table and giving control to the boot sector of the active partition. If there is no active partition, an error message is displayed.

Root Directory

The FAT file system uses a root directory (represented in operating systems that use this type of file system by the command C:\), which must be located at a specific location on your hard disk. This root directory stores information about the subdirectories and files it contains.

In the case of a file, it stores:

• • File name
  • File size
  • The date and time the file was last modified
  • File attributes
  • The cluster number where the file is located

Partitioning a Hard Disk

Every hard disk is a different physical drive. However, operating systems do not work with physical units directly, but with logical drives. Within the same physical disk drive, there can be multiple logical drives. Each of these units is a logical hard disk partition.

.- Both partitions and directories organize data structures within a hard disk. However, there are important differences:

1st) Partitions are divisions of fixed size on the hard disk. Directories are divisions of varying size within a partition. 2nd) Partitions hold a contiguous group of cylinders on the hard disk (for safety). Directories often have information scattered across the entire partition. 3rd) Each disk partition can have a file system (potentially a different operating system), while all directories within a partition share the same file system.

At a minimum, you must create one partition for each hard drive. A hard disk can only have 4 primary partitions (including the extended partition, if any). Existing partitions must be registered in a 4-entry partition table located in the first sector of every disk drive. Of these 4 entries, any may not be used (a non-partitioned hard drive as shipped from the factory), or one, two, three, or four may be used. In any of these latter cases (even when only one partition)