Magnetic disk drives (floppy and hard drives) provide a permanent data storage. Capacities today are enormous compared to just a couple of years ago. Even so the basic technology remains the same, they are just bigger with faster access times. The default values given do not represent actual values used today but chosen to explain the principles involved. The fastest transfer rates of today’s hard drives are more than 100Mbytes/sec. Each disk consists of tracks to store the data, arranged within sectors. Disk formatting sets up the tracks and sectors. One track within one sector is the smallest amount of data that can be accessed. This group of bits is transferred to the CPU RAM, modified if necessary and written back to the disk. Individual bytes on the disk are not accessible. To minimise access times, i.e. ‘disk rotation and seek time’, data is arranged on the disk in cylinders, not as you might expect, all in one part of a single disk.
Combinations of sector tracks (at various locations) are known as clusters and will hold the data for a complete file. A file allocation table (FAT) is a map of how the data is organised on the disk. Files don’t follow consecutive locations, but are split up to fit in the next available free area. To see this in action run the ‘disk defragmenter’ program, in the Windows System Tools folder. Erasing a file does not actually delete the data on the disk it just modifies the FAT, which is the reason data can often be recovered successfully providing the released disk space has not been overwritten