Hard disks are written and read magnetically. Optical discs are written with lasers. HD refers to Hard Disk, typically a metal box containing non-magnetic platters coated with magnetic -ferrous- particles or vapor molecules, spinning like a phonograph record as heads with coils of wire almost touching it's surface can magnetize or demagnetize by quick current impulses, or sense (read) by imparting an electronic voltage,when crossing over magnetized bits of the platters . Polarizing the head coils can force the particles or molecules on the platter to 'align' like a compass points to north pole in specific direction. To create modern hard drive platters, A ferrous metal is deposited on discs similar to CDs in a process known as 'vapor deposition', where the metal is vaporized by heat and directed by static electricity fields to attract the vapor to the non-ferrous metal disk in an even application, like painting a car. This higher density of smaller particles of metal and better understand of magnetic fields and development of special materials, has resulted in very small shaped magnetic field heads, which can write, depolarize, and read, amazingly small bits of platters. The smaller the bit, the larger the drive capacity can be with the same size and number of platters. Heads in pairs -under and over each platter - are on a single arm which swings from outer rim to inner shaft controlled by a precise step motor that advances inward or out in 'ticks' that represent one 'track' or revolution of data possibility. These disks -or platters- spin at from 4800 to 1000 revolutions per minute -most now about 7500rmp -and there may be one or many platters in one HD, but all heads in consumer hard drives are attached to same servo arm and sandwich each platter. In this way data is multiplied, as each 'read' will read a bit from each side of each platter at same time. The question is how they are written - magnetically. When you type a key, you enter a byte, or 8 bits, of data. Remember a bit is what one head can write the smallest on a platter. A byte was one character of 256 possible ones. Modern computers can use more characters than 256, but the old model is best to understand how data is written to a drive. Data in computer is entered and stored as ascii coded binary. One binary character contains 8 bits, which can be visualized by segments of an old LED display. Seven segments are 'lit' or not, to form letters and numbers. The eighth bit is used for parity -to check for errors in transmitting or receiving packets, sectors, or other quantities of data. Similar parity mathematical logic algorithms are used to check transmission of all data on internet. That, and the interface between different computers and interfaces like IDE SATA 1394 USB, are reason for chips on a card inside the hard drive. The parity checking is internal on the drive, and many can even 'block' sectors on the platters (what the discs in a hard drive are typically called) which are damaged or had insufficient field strength in manufacture, so they are no longer read or written in hard drive operation. When you type a character on the keyboard, a byte - eight bits - is written to computer RAM memory. That is typically saved as a "pop string" which is taken as a group when you select it and put it somewhere. The hard drive has addresses, global -text, and number, which the heads when searching for a place to find or store information will know what is available by the address codes written to the HD. When something is deleted, only the address is zeroed, which means it may then be written over when new data needs to be stored. Unless a 'secure delete' is performed which takes up to hundreds of times longer, because each bit must be written and erased -sometimes many times, by random or purposeful (in high security operation) data in order to not leave a 'latency' pattern. Latency is when the magnetic field after a bit is erased, still contains enough extra magnetic value, that heads adjusted in sensitivity may be able to determine the previous data well enough to read it, like a screwdriver after touching a magnet will be able to hold a screw to it's tip, but even after an AC coil is turned on around it to depolarize, may still be able to roll a screw on a table. Short answer -= When you strike a key on your keyboard, the 'card' inside the keyboard puts out 8 bits as one character on wires to your computer. It is a binary character, so it has eight ones and/or zeros. The first binary is added to the second and so on, like this. 0000001 is one. Zero is a value in binary(base2 number system), - not a multiplier- as in the decimal(base 10) you are familiar with. 0000011 is three; 2nd power of the 2nd from last one, plus the single (last)one. So 0000111 is 7. Power of 2 again. Reading from RIGHT TO LEFT, 1 to second power is one, 2 (second place one) , second power of 2 is 2, third is 4, 8 16, and so on. Power of 2 is applied to each binary one or zer