A hard disk begins life as a circular disk with an undifferentiated magnetic coating. To be useful, that hard disk is formatted.
Formatting in its simplest form means that the disk is magnetically divided into cylinders and sectors. The "cylinders" are actually concentric circles radiating out from the center that would look a lot like a bulls eye with many rings. Each of those cylinders is divided into segments called sectors. Each sector is addressed by its cylinder number and its sector number in the cylinder.
The sector is the smallest and (arguably) the largest usable space on the disk. The computer will not put two files in one sector even if both of them together will fit in it.
On the other side of size, the sector is the largest usable space as well. When a file needs more than one sector of space, the first sector is filled except for a few bytes. The computer then goes and finds the next available sector, writes whatever it can to that new sector, and puts the address of the new sector on those remaining few bytes on the first sector -- that address is called a link.
The computer continues grabbing sectors, filling them, and writing links until the file is completely written to disk. If the file is deleted, then all of the sectors are returned to the cache of unused sectors. If the file is shortened, then the new end of the file is found and any unnecessary sectors are returned to the unused sector cache. If the file is extended, the computer grabs whatever new sectors are needed from where ever they are available and builds the links as necessary.
The disk drive requires two kinds of physical motion. First the hard drive disk is spinning. Oh! it is spinning quite fast but it is a motion well below the speed of electronic memory. The second motion is the head. That is similar to the head in your tape drive but much more exact. It reads variations in the magnetic media to determine what the data is. The spinning of the disk allows it to read different sectors in the same cylinder. To read different cylinders, the head must move backwards and forwards over the disk from cylinder to cylinder and reading sectors as they pass beneath the head.
It really is a simple system that works quite well. However, it is also the main hindrance to speed on the standard home computer because it is the only thing on it that depends on physical motion. You see, CDs, DVDs, floppies, and hard drives all basically work the same way.
