Hard Disk Drive

History of Hard Disk:

The hard disk drive was initially developed as data storage for the IBM 305 RAMAC computer system. IBM announced HDDs in 1956 as a component of the IBM 305 RAMAC system and as a new component to enhance the existing IBM 650 system, a general-purpose mainframe. The first IBM drive, the 350 RAMAC in 1956, was approximately the size of two medium-sized refrigerators and stored five million six-bit characters (3.75 megabytes) on a stack of 50 disks.

How does Hard Disk Work ?

Inside the hard drive are sectors located on tracks, which are stored on rotating platters. These platters have magnetic heads that move with an actuator arm to read and write data to the drive.


The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, FireWire, and differential SCSI.

Components of Hard Drive:


The platters are the circular discs inside the hard drive where the 1s and 0s that make up your files are stored. Platters are made out of aluminum, glass or ceramic and have a magnetic surface in order to permanently store data. On larger hard drives, several platters are used to increase the overall capacity of the drive. Data is stored on the the platters in tracks, sectors and cylinders to keep it organized and easier to find.

The Spindle

The spindle keeps the platters in position and rotates them as required. The revolutions-per-minute rating determines how fast data can be written to and read from the hard drive. A typical internal desktop drive runs at 7,200 RPM, though faster and slower speeds are available. The spindle keeps the platters at a fixed distance apart from each other to enable the read/write arm to gain access.


Figure 22 – Hard Disk Drive with all it’s components

The Read/Write Arm

The read/write arm controls the movement of the read/write heads, which do the actual reading and writing on the disk platters by converting the magnetic surface into an electric current. The arm makes sure the heads are in the right position based on the data that needs to be accessed or written; it’s also known as the head arm or actuator arm. There is typically one read/write head for every platter side, which floats 3 to 20 millionths of an inch above the platter surface.


The actuator or head actuator is a small motor that takes instructions from the drive’s circuit board to control the movement of the read/write arm and supervise the transfer of data to and from the platters. It’s responsible for ensuring the read/write heads are in exactly the right place at all times.

Other Components

As well as the casing on the outside of the hard disk that holds all of the components together, the front-end circuit board controls input and output signals in tandem with the ports at the end of the drive. No matter what the type of drive, it has one port for a power supply and one port for transferring data and instructions to and from the rest of the system.

Some related terms:

Sector & Track: A sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user-accessible data, traditionally 512 bytes for hard disk drives (HDDs) and 2048 bytes for CD-ROMs and DVD-ROMs. Newer HDDs use 4096-byte (4 KiB) sectors, which are known as the Advanced Format.

Geometrically, the word sector means a portion of a disk between a center, two radii and a corresponding arc (see Figure 1, item B), which is shaped like a slice of a pie. Thus, the disk sector (Figure 1, item C) refers to the intersection of a track and geometrical sector.

Seek time :- The seek time is the time it takes a specific part of a hardware’s mechanics to locate a particular piece of information on a storage device. This value is typically expressed in milliseconds (ms), where a smaller value indicates a faster seek time. Seek time is often called access time, but in reality the access time is bit longer than the seek time because there exists a small latency period between finding data and then actually accessing it.


Figure 13 – Disk structure: A=Track, B=Geometrical sector, C=Track sector, D=Cluster

File System – FAT32 & NTFS :- FAT32 is the older of the two drive formats. FAT32 is the most common version of the FAT (File Allocation Table) file system created back in 1977 by Microsoft. It eventually found its way on the IBM PC’s PC-DOS in 1981, and carried over to MS-DOS when that became a standalone product. FAT had been the standard format for floppy disks and hard drives all through the DOS years, and versions of Windows up to and including Windows 8.

NTFS (New Technology Files System) is the newer drive format. Microsoft introduced NTFS in 1993, as a component of the corporate-oriented Windows NT 3.1 and then Windows 2000, though it didn’t become common on consumer PCs until Windows XP in 2001. Windows 7 and 8 default to NTFS format on new PCs.

Low-level and high level formatting :-

Low-level formatting :- It is the process of outlining the positions of the tracks and sectors on the hard disk, and writing the control structures that define where the tracks and sectors are. This is often called a “true” formatting operation, because it really creates the physical format that defines where the data is stored on the disk.

The first time that a low-level format is performed on a hard disk, the disk’s platters start out empty. That’s the last time the platters will be empty for the life of the drive. If an LLF is done on a disk with data on it already, the data is permanently erased.

High Level Formatting:– After low-level formatting is complete, we have a disk with tracks and sectors–but nothing written on them. High-level formatting is the process of writing the file system structures on the disk that let the disk be used for storing programs and data. High-level formatting is done after the hard disk has been partitioned, even if only one partition is to be used.

Difference between High Level and Low Level Formatting:– The distinction between high-level formatting and low-level formatting is important. It is not necessary to low-level format a disk to erase it: a high-level format will suffice for most purposes; by wiping out the control structures and writing new ones, the old information is lost and the disk appears as new. (Much of the old data is still on the disk, but the access paths to it have been wiped out.) Under some circumstances a high-level format won’t fix problems with the hard disk and a zero-fill utility may be necessary.

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