A+ (2006-07) / Lessons

Removable Storage

Floppy disks

The 8" floppy disk was created in the late 1960s by IBM.  It consisted of a plastic disc with a ferromagnetic coating that could be selectively magnetised, just like audio tape.  They soon added a firm, square plastic envelope to protect the disk from dust.  The eight inch disk originally stored approximately 100 KB of data but by 1977 this had increased to around 1 MB.

The 5" floppy disk was introduced in 1976: it was essentially a smaller version of the 8" disk and stored 90 KB of data.  The later double-sided, double-density disk would hold 360 KB and high density versions 1.2 MB.

Early PCs had 5" drives and the floppy interface found on modern machines is still compatible with these devices, if the correct cable is used.

The 3" floppy disk was created by Sony in 1983.  It had a rigid case and metal sliding door to protect the disc surface, with capacities of 720 KB  or 1.44 MB (high density).

A number of other floppy standards existed, such as the 3" disks used with Amstrad's PCW wordprocessor and home computers, but the 3" floppy became the popular standard for 20 years.

Floppy replacements

By the mid 1990s it was clear that the floppy disk did not have the capacity for the quantity of data that people needed to move quickly and easily.  A number of different systems were designed to succeed the floppy disk but most failed to gain mass market.

Zip disk

Iomega's zip disk could store 100 MB of data and became a popular format among people doing graphics and audio work, where single files would typically exceed 2 MB size.

Later revisions to the zip drive increased capacity to 250 MB and 750 MB.

NOTE: the "zip" in zip disk is simply a brand name.  It should not be confused with "zipping" data — a method of compressing multiple files into a single compact file called a "zip archive" or "zip file".

Jaz drive

The Iomega Jaz drive was a removable hard disk available in 1 GB or 2 GB sizes.  The hard drive cartridge resembles a data cassette.

LS-120

The LS-120 disk drive could read both 3" floppy disks and special LS-120 disks, with a capacity of 120 MB.  They were far less popular than zip disks but were sometimes used in the medical profession for storing images.

Solid state devices

USB Flash drive

The most popular form of removable storage nowadays is the USB Flash drive, also known as a thumb drive.  These devices contain electrically-erasable permanent memory called Flash memory and come with capacities ranging from 64 MB to 2 GB.

The devices plug into a PC's USB port and therefore require a device driver.  Windows XP has suitable drivers built in for many Flash drives but sometimes a specific driver will be required, especially if the drive offers encryption of data.

Some models have built-in MP3 players.

Memory cards

There are many different types of memory card used by digital cameras and PDAs.  With an appropriate adaptor they can also be read by PC or laptop.

These cards have different capacities and access speeds and vary dramatically in price.  It is always a good idea to investigate the specification of the card before committing to a particular format.

External hard drives

External hard disk drives can be used as a removable storage medium.  These are usually too power-hungry to work direct from USB and require a separate power supply.

Optical storage

CDs

Compact Discs come in a range of different types and formats.  Each type must conform to a published specification known as a "Rainbow Book".

Data on a CD is recorded on a single spiral track that starts at the centre of the disc and is recorded as a sequence of 1s and 0s, encoded as pits (indentations) and lands.

Information is read at a constant rate, which means that data flows faster as the scanning laser moves towards the outer edges of the disc.

The data layer is coated and overlaid with a printed label.

Types of CD

Digital audio (CD-DA) discs follow a standard called the Red Book.  It specifies that the audio data on the disc is encoded as 16-bit binary samples taken 44,100 times per second.  This gives up to 80 minutes of playback.

CD-ROMs (Yellow Book) contain computer data only.  A variant of audio CD called Enhanced CD (Blue Book) allows computer data to be stored alongside sound data on CD-DAs.

Video CDs (White Book) contain 74 minutes of MPEG1 video, with stereo sound.  Although commercial VCDs were released in the early 1990s the format did not take off until the rise of CD-recorders ten years later.  In the Far East, pirate copies of movies were often distributed on VCD.

This led to the development of Super VCD (SVCD), an unofficial variation of VCD supporting MPEG2 audio and surround sound — in essence a mini-DVD.

Kodak's Photo CD format (Beige Book) was designed to hold high quality photographs and thumbnails in a form compatible with a range of equipment.

Super Audio CD (SACD) (Scarlet Book) is a dual-layer CD that supports surround sound.  It requires a special player; however, some SA CD discs use one layer for conventional CD audio and will play in standard CD players.

Recordable CDs

CD-R has a photosensitive dye on the spiral track.  This dye is chemically altered by writing laser to change its colour, absorbing playback laser light and creating the impression of pits and lands.  CD-R is a write-once, read-many (WORM) medium.

CD-RW works in a similar way but uses a metallic alloy instead.  This alloy is chemically altered by the writing laser, making it refract light at a different angle.  This process can be cycled up to 1000 times.

The effective difference between "lands" and "pits" on a CD-RW is less than that of CD-R; therefore many CD players cannot read CD-RWs.  Most DVD players, however, are designed to read CD-RW discs.

DVDs

DVD — Digital Video Disc (later changed to Digital Versatile Disc) — works in a similar way to CD but uses a denser arrangement of pits and lands to store approximately 4.5 GB of data.

It is possible to lay a second, semi-opaque, surface onto the disc.  The focus of the laser can be adjusted to read either layer, creating a dual-layer disc, capable of holding 9 GB.

In addition, the specification allows for this to be repeated on the other side of the disc (omitting a label, of course) to create a dual-layer, double-sided disc.  This kind of disc can store up to 18 GB of data.

Minus versus plus

There are two standards for recordable DVD:

DVD-R and DVD-R/W

DVD+R and DVD+R/W

There is little technical difference between the two, but the '–' formats were designed originally for video data and are more widely compatible with TV DVD players.  However, DVD+RW can also be erased in sections without wiping the whole disc.  Dual-layer recordable discs are only available in '+' format

Drives marked DVD±R and DVD±R/W are capable of working with both types of DVD.

DVD-RAM

DVD-RAM was designed for video recording.  It is not as popular as DVD±R(W) and can only play on compatible DVD players.

High Definition DVDs

Blu-Ray and HD-DVD offer greater storage than current DVD technology, with between 25 GB and  200 GB capacity.

Drive speeds

Advances in technology have made it possible to read data from discs at a higher and higher rate.  Be careful to check that recordable media will work with the speed of drive that you are using.

Care of CDs and DVDs

Because the layers are very thin even a minor scratch can damage data.  The label is the most dangerous area because this is least protected.  Pencils and ballpoint pens should not be used to write on labels; in addition, some kinds of permanent marker can chemically react with the label and layers over time.  It is recommended that you use a special CD / DVD marker pen.

Prolonged exposure to sunlight causes warping and fading.  You should store CDs between 20°C and 50°C and in 5% to 90% humidity.  Motor vehicles are bad places to leave discs because they are subject to extremes of temperature during sunlight or winter.

Manufacturers usually recommend that you clean fingerprints and marks from the underside by wiping in a straight line from centre hole to outside edge with a lint-free cloth.  DON'T wipe in a circular motion and certainly don't use solvent or abrasive cleaners.  Isopropyl alcohol (IPA) can be used but it should be applied sparingly to avoid tarnishing the surface.

Handle CDs only by their edges to prevent fingerprinting and store discs in their cases to avoid scratches