Next came paper tape where the individual cards were replaced with a single continuous tape with holes punched in it. It also was read only and unlike the cards, it could not be re-arranged. However, if you dropped it, it was not a disaster. Also, it had to be read sequentially.
This in turn was replaced by magnetic tape. It had many of the same properties of paper tape. However, it could be re-written as a whole and with some care, even smaller portions could be re-written. Current magnetic tape devices can read/write data very quickly and is still considered an inexpensive way to backup and transport very large collections of data or program.
The magnetic media is placed on one or both sides of the magnetic disk or platter and one or more platters can be stacked together to create a larger storage area. The magnetic media on the surface of the platter is then organized into concentric parallel tracks. This can be done on each surface of the platter or platters. The collection of tracks that all fall on the same location on each platter surface is called a cylinder.
Each suface on the platter(s) has its own read/write head. It is now possible to store data on one of the tracks, note the cylinder and head (thus the exact track) and go directly to that location.
However, a track can hold quite a lot of information. Additionally, there has to be some way of determining where on the track the data begins. To accomplish this, the track is divided into sectors with ids. The data to be stored on the storage device is divided into blocks sized to fit a sector. A sector is written to or read from sequentially. When data is stored, the cylinder, head, and sector (chs) is recorded and can be used to go directly back to that location to access or change the data.
The size of the sector determines how quickly and directly a particular piece of data can be accessed. If the sector is very small, it can be quicky with very little access to unneeded data. However, the location of every piece of data and its sector must be recorded. So, there is now "meta-data" pointing to data and the meta-data must also be stored.
The solution to this is to make the sectors larger. However, if the sectors are too large, they start to behave like sequential magnetic media. Currently, sectors of 512 bytes offer the best comprimise between quick access and efficient tracking. They are large enough to to create an efficient index system and they are small enough to be read sequentially as fast as the system electronics permits.
To achieve the high data access, called throughput, the discs must be spun at very high speeds. Current speeds available are 3600 rpm, 7200 rpm and over 10000 rpm. Because of this speed, the read/write heads are designed to not touch the surface of the platter, but rather float or fly on a cusion of air.
On a per byte basis, magnetic disks (hard drives) are very cost effective for their access time and read/write capabilities.
However, they are not cheap. An alternative that has been developed is the compact disk. Based on the use of optical media rather than magnetic, the cd was originally developed for recording digitized sound. It shines a small infra-red laser on the disk's surface and notes when the beam stops or starts refecting back. Because the data stored is digitized, it was very easy to convert the software/hardware reading the data to recognize computer data.
A cd is capable of holding 650 MBytes of data.