How is information stored magnetically on a hard drive?

Answer 1

A hard drive is "hard" and "driven" mechanically. At one time computers were limited to tapes, then floppy disks then hard disks.

Actually, that is not correct. The various dates of introduction of the magnetic bulk data storage technologies is as follows:

1. metal tape, 1951 (very heavy, a small reel of tape weighed over 10 pounds but did not bend or stretch left residue in the drive requiring frequent cleaning)
2. plastic tape, 1952 (much lighter and soon made metal tape obsolete but could bend and stretch)
3. hard disk, 1956 (first drive had 50 platters 24 inches in diameter in a stack 48 inches tall, the seek time was 0.8 seconds and the capacity was 5000000 six bit BCD characters; disk coating was a slightly modified formulation of the paint used on the Golden Gate Bridge in San Francisco, CA)
4. floppy disk, 1971 (designed only for loading microcode into IBM System/370 computers at initial startup, never intended to be kept in the drive longer than a few minutes, original capacity 80000 bytes)

These all work in a similar manner. The trouble with tapes and floppy disks is that they bend. This distorts the signal stored on them limiting the speed information can be written.

A hard disk, on the other hand, is made of metal (aluminum) and doesn't bend so much meaning the signal is a lot more consistent.

There are several disks (flat, circular pieces of metal, called platters) inside a hard drive that spin quite fast. These disks have a fine coating of a substance which can be magnetised easily such as Iron oxide or Chromium dioxide. Next to these disks there is a read/write "head". This consists of a small bead of ferrite material with a coil wrapped around it. As the hard disk spins, air close to the disk keeps the head from actually coming in contact with the disk but it is close enough to do its job.

When small pulses of electricity are passed through the coil around the ferrite bead, it causes a magnetic field. This magnetic field, in the bead, magnetises the surface of the disk. A simple way of recording data is to record a magnetic pulse for a 1 and a reverse pulse for a zero. 1s and 0s are part of the binary number system which is used throughout the computer. It is only converted back and forth between binary and decimal when you type things in or read them on the screen. Even then, it is really still binary in groups called bytes (8), words (16), double words (32), quad words (64) tbytes (80) and so forth.

These days there are much better and faster ways of recording data than simple pulses for 1 and 0. There are a number of systems including, NRZ NRZI, FM, MFM, dibits and many more.