HARD DISC :- A hard disk is a sealed unit that a PC uses for nonvolatile data storage. The hard drive is used to store crucial programming and data. A hard disk drive contains rigid, disk-shaped platters, usually constructed of aluminum or glass. A hard disk uses round, flat disks called platters, coated on both sides with a special media material designed to store information in the form of magnetic patterns. The platters are mounted by cutting a hole in the center and stacking them onto a spindle. The platters rotate at high speed, driven by a special spindle motor connected to the spindle. Special electromagnetic read/write devices called heads are mounted onto sliders and used to either record information onto the disk or read information from it. The sliders are mounted onto arms, all of which are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator. A logic board controls the activity of the other components and communicates with the rest of the PC. Each surface of each platter on the disk can hold tens of billions of individual bits of data. Each platter has two heads, one on the top of the platter and one on the bottom, so a hard disk with three platters (normally) has six surfaces and six total heads. Each platter has its information recorded in concentric circles called tracks. Each track is further broken down into smaller pieces called sectors, each of which holds 512 bytes of information. Hard Disk – Platters and Media :- • Every hard disk contains one or more flat disks that are used to actually hold the data in the drive. • These disks are called platters (sometimes also "disks" or "discs"). • They are composed of two main substances: – a substrate material that forms the bulk of the platter and gives it structure and rigidity, – a magnetic media coating which actually holds the magnetic impulses that represent the data. • Hard disks get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible "platters" (actually, they aren't usually even called platters when the material is flexible.) Hard Disk – Tracks and Sectors :- • Each platter is broken into tracks--tens of thousands of them--which are tightly-packed concentric circles. • Track is one of the many concentric circles that holds data on a disk surface. • A track holds too much information to be suitable as the smallest unit of storage on a disk, so each one is further broken down into sectors. • A sector is normally the smallest individually-addressable unit of information stored on a hard disk, and normally holds 512 bytes of information. – The first PC hard disks typically held 17 sectors per track. – Today's hard disks can have thousands of sectors in a single track, and make use of zoned recording to allow more sectors on the larger outer tracks of the disk. A platter from a 5.25" hard disk, with 20 concentric tracks drawn over the surface. This is far lower than the density of even the oldest hard disks; even if visible, the tracks on a modern hard disk would require high magnification to resolve. Each track is divided into 16 imaginary sectors. Older hard disks had the same number of sectors per track, but new ones use zoned recording with a different number of sectors per track in different zones of tracks Hard Disk – Read & Write Head :- • The read/write heads of the hard disk are the interface between the magnetic physical media on which the data is stored and the electronic components that make up the rest of the hard disk (and the PC). • The heads do the work of converting bits to magnetic pulses and storing them on the platters, and then reversing the process when the data needs to be read back. Hard Disk Read/Write Operation :- • Older, conventional (ferrite, metal-in-gap and thin film) hard disk heads work by making use of the two main principles of electromagnetic force. – Write : applying an electrical current through a coil produces a magnetic field;. The direction of the magnetic field produced depends on the direction that the current is flowing through the coil. – Read : that applying a magnetic field to a coil will cause an electrical current to flow; this is used when reading back the previously written information. • Newer (MR and GMR) heads don't use the induced current in the coil to read back the information; they function instead by using the principle of magnetoresistance, where certain materials change their resistance when subjected to different magnetic fields. Hard Disk :- – An MR head employs a special conductive material that changes its resistance in the presence of a magnetic field. As the head passes over the surface of the disk, this material changes resistance as the magnetic fields change corresponding to the stored patterns on the disk. A sensor is used to detect these changes in resistance, which allows the bits on the platter to be read. – MR technology is used for reading the disk only. For writing, a separate standard thin-film head is used. This splitting of chores into one head for reading and another for writing has additional advantages. • Ferrite vs MR Head – The use of MR heads allows much higher areal densities to be used on the platters than is possible with older designs, greatly increasing the storage capacity and (to a lesser extent) the speed of the drive. – allows the use of weaker written signals, which lets the bits be spaced closer together without interfering with each other, improving capacity by a large amount. Extreme closeup view of a ferrite read/write head. The head is at the end of the slider, wrapped with the coil that magnetizes it for writing, or is magnetized during a read. Closeup view of an MR head assembly. Note that the separate copper lead wire of older head designs is gone, replaced by thin circuit-board-like traces. The slider is smaller and has a distinctive shape. The actual head is too small to be seen without a microscope. Hard Disk Tracks, Cylinders and Sectors :- All information stored on a hard disk is recorded in tracks, which are concentric circles placed on the surface of each platter, much like the annual rings of a tree. The tracks are numbered, starting from zero, starting at the outside of the platter and increasing as you go in. A modern hard disk has tens of thousands of tracks on each platter. Data is accessed by moving the heads from the inner to the outer part of the disk, driven by the head actuator. This organization of data allows for easy access to any part of the disk, which is why disks are called random access storage devices.Each track can hold many thousands of bytes of data. It would be wasteful to make a track the smallest unit of storage on the disk, since this would mean small files wasted a large amount of space. Therefore, each track is broken into smaller units called sectors. Each sector holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction. Clusters Each partition on your hard disk is subdivided into clusters. A cluster is the smallest possible unit of storage on a hard disk. The size of a cluster depends on two things: 1. The size of the partition 2. The file system installed on the partition Regardless of the size of your partition, the maximum number of clusters remains the same. For example, in a standard FAT file system, a partition can contain no more than 65,536 clusters. Because of this limitation, cluster sizes vary depending on partition size. Basically, a small partition uses small cluster sizes while a large partition uses large cluster sizes. In a world where bigger is better and size does matter, you might assume that a big hard drive with huge cluster sizes is the way to go. However, this isn’t the case. As you’ll recall, clusters are the smallest unit of storage space on a hard disk. This means that you can’t share a cluster among multiple files. If you have a tiny file and a huge cluster, the portion of that cluster unused by the file is wasted. To understand the above statement, it’s necessary to look at some real numbers. In a standard FAT file system, the smallest size that a cluster can be is 2 KB. Therefore, if you save a 1-byte file on a hard disk using a 2-KB cluster size, you’ll actually use the entire 2 KB (2,048 bytes). So, you could potentially waste over 2,000 times more space than you’d use if you had a hard disk full of 1-byte files. What makes this fact even scarier is that, as I mentioned before, 2 KB is the smallest size that a cluster can be. However, don’t forget about that 65,536-cluster limit. This means if you want to use 2-KB clusters, your partition size can be no more than 128 MB (2-KB clusters multiplied by 65,536 total clusters equals 134,217,728 bytes, or 128 MB). If you want to have a FAT partition larger than 128 MB, the cluster size will have to grow. As the size of your partition increases, the cluster sizes double to accommodate it. Below is a chart that outlines the number of clusters required to accommodate various partition sizes.