this section will be looking at the various forms of secondary storage devices (media). For each device you should be familiar with the following details: Seek time- The average time taken from requesting data to starting to read the requested data Capacity - The amount of data it is possible to store on a medium Write type - Whether it is read only, write only, or readable and writable Cost - How much it costs per megabyte Access type - Whether it uses Random Access or Serial Access

sequential access requires all memory locations before the one sought to be read, before reaching it. Random access memory allows you to jump directly to the memory location you are seekingPay special attention to devices with anorangebackground, you need to be able to describe exactly how they work! 1Magnetic media 1.1Hard disk 1.2Magnetic Tape drive 2Optical media 2.1CD-ROM 2.2CD-R 2.3CD-RW 3Solid-state memory 3.1USB (memory stick) Flash Drive 3.2Memory cardsMagnetic media[edit]Magnetic mediastores data by assigning a magnetic charge to metal. This metal is then processed by a read head, which converts the charges into ones and zeros. Historically, magnetic media has been very popular for storing programs, data, and making backups. It looks set to continue in this role for some time. However, solid state technology is starting to be used more and more, storing programs and data on new devices such as mobile phones and cameras.Magnetic media

DeviceSize

Hard DiskUp to 8 Terabytes

Magnetic TapeUp to 2 Terabytes

Hard disk[edit]

Video of exposed hard disk drive (HDD)Hard disks are usually found inside computers to store programs and data. They are increasingly cheap and more and more companies are using them to back things up. Hard disks can vary in physical size with some disks getting as small as your thumb. More closely packed platters, greater density of data on each platter to allow for more tracks and cylinders and the ability to write smaller magnetics spots have all been developments in the design of hard disks to increase their storage capacity. The capacity of a commercial disk is currently up to about 4 terabytes allowing users to read and write to them. They are constructed from several key components: Platter- Metallic disks where One or both sides of the platter are magnetized, allowing data to be stored. The platter spins thousands of times a second around the spindle and spins continuously when in operation. There may be several platters, with data stored across them. The disk is divided into tracks and sectors with data represented by magnetising spots on the the disk. Head- The head reads magnetic data from the platter. For a drive with several platters there may be two heads per platter allowing data to be read from top and bottom of each Actuator Arm- used to move the read heads in and out of the disk, so that data can be read and written to particular locations and you can access data in a Random fashion, you don't need to read your way through the entire disk to fetch a particular bit of information, you can jump right there. Seek time is very low. Power connector- provides electricity to spin the platters, move the read head and run the electronics IDE connector- allows for data transfer from and to the platters Jumper block- used to get the disk working in specific ways such asRAID

For the exam you must be able to explain how a hard disk works:1. The platters spin around the spindle2. data is requested to be read from a particular area of a platter3. the actuator arm moves the read head to that track4. Once the data sector that is required has spun around and under the read head, data is read5. Read data is sent from the IDE connector to main memory

Description of a hard disk platterWriting data is very similar:1. The platters spin around the spindle2. data is sent to the hard disk using the IDE connector3. the actuator arm moves the write head to the track that will be written to4. Once the data sector that is required has spun around and under the write head, data is written to the platterProsFast seek timesRandom accessHigh capacities possibleLow cost per megabyte

ConsVery susceptible to damage from physical shocks

Magnetic Tape drive[edit]

DDS tape drive. Above, from left right:DDS-4 tape (20 GB), 112mData8tape (2.5 GB),QICDC-6250 tape (250 MB), and a 3.5"floppy disk(1.44 MB)Increasingly obsolete, the tape has been a medium to deliver software and back up data since the early days of computing. Nowadays they are used mostly for corporate backing up and archiving of data. Tapes are sequential data stores, meaning that if you had information stored at the end of the tape you would have to wind your way through the entirety of the tape before you could read it. There is no random access like with a hard disk! Tapes can be several terabytes in size and reading and writing can be very fast as long as you read or write continuous sections of the tape at once.ProsFastHigh capacityCheap per megabyte

ConsSerial read and write capabilities

Optical media[edit]Optical media works by creating a disc with a pitted metallic surface. There are several different types of disk out there ranging from 650 MB to 128 GB, with the pits and lands getting closer together for higher volume disks. The principle behind how each of them works is the same.

pitted surface visible on the surface of a CD. Massively zoomed in!Optical media

DeviceTypeSizeImage

CD-ROM CD-R CD-RW Read Only Write once then Read only re-Writable650 - 900 MB

DVD-ROM DVD-R DVD-RW DVD-RAM Read Only Write once then Read only re-Writable re-Writable4.7 - 9.4 GB

Blu-ray (BD) disc HD DVD (obsolete)Re-Writable and Read Only versions available. Uses a blue laser, that is able to recognise smaller pits and lands, which allows for the pits and lands to be more closely packed, and so store more data25 - 128 GB

CD-ROM[edit]

close up of the surface of a CD being readA CD-ROM is a metal disc embedded into a plastic protective housing. Each disc has to be 'mastered'; this is the process of creating the CD and placing the data on it. CDs are WORM (Write Once, Read Many) media; this refers to the fact that once they have been mastered, there is no way to change the data on them.Reading from a CD-ROM1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data2. A low-powered laser is shone on the metallic surface and the reflection is captured in a photodiode sensor, the lands reflect differently to the pits, meaning it can tell the difference between a 1 and a 03. The disc spins and the laser follows the track4. The binary data (the 1s and 0s) are put together and the CD-ROM has been readProsCheapData cannot be written over by the consumerConsSlow seek timeData degrades with time, discs from 20 years ago might not work!Can only be written to with a very high powered laser, which is not usually available in home computersData cannot be written over

CD-R[edit]The CD-R is made of a reflective metal disk with a layer of (usually green, opaque) dye on top.Writing to a CD-R1. A single track runs in a spiral pattern from the centre of the disc to the outside.2. A high-powered laser is shone onto the CD-R, changing the transparency (permanently) of the dye above. The transparent and opaque parts represent binary 1s and 0s3. The disc spins and the laser follows the track, putting the binary data onto the CD-R in a spiral track4. The data has been writtenReading from a CD-R1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data2. A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The opaque dye will reflect differently to the transparent dye (which would just reflect the metal underneath it), meaning it can tell the difference between a 1 and 03. The disc spins and the laser follows the track4. The binary data (the 1s and 0s) are put together and the CD-R has been readProsCheapCan be written to using a conventional home computerConsSlow seek timeData degrades with time, discs from 20 years ago might not work!Data cannot be written over

CD-RW[edit]The CD-RW is made of a reflective metal disk with a layer of a special ('phase change') metal on top.Writing to a CD-RW1. A single track runs in a spiral pattern from the centre of the disc to the outside.2. A high-powered laser is shone onto the CD-RW. Depending on whether this is very high powered or heats at a slightly lower temperature, the top layer of metal cools differently. These will result in different amounts of reflectivity, which represent the 1s and 0s.3. The disc spins and the laser follows the track, putting the binary data onto the CD in a spiral track4. The data has been writtenReading from a CD-RW1. A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data2. A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The different ways the metal has cooled reflect different amounts, meaning it can tell the difference between a 1 and 03. The disc spins and the laser follows the track4. The binary data (the 1s and 0s) are put together and the CD-RW has been readProsCheapCan be written to using a conventional home computerConsSlow seek timeData degrades with time, discs from 20 years ago might not work!Data can be changed after writingNot all CD players (mostly older ones) can read CD-RWs, as opposed to CD-ROMs and CD-Rs

Solid-state memory[edit]Solid-state memory

DeviceDescription

USB flash driveUp to 256 GB

Memory cardUp to 256 GB

USB (memory stick) Flash Drive[edit]Internals of a typical USB flash drive

1USB Standard-A plug

2USB mass storage controller device

3Test points

4Flash memory chip

5Crystal oscillator

6LED

7Write-protectswitch (Optional)

8Space for second flash memory chip

USB Flash drives are solid state, that means that there are no moving parts. This is very useful for seek times as we don't have to wait for mechanical movement, meaning seek time is very low and it allows for fast Random Access Memory. Flash drives can be set to read only mode, but they will always allow for reading and writing. The size of flash drives is not as great as a Hard Disk and they are generally much more expensive per megabyte1. put drive into USB socket2. USB driver loads, providing the computer with code on how to read and write from the USB3. The USB is read, giving information on the file and folder structure (File Allocation Table) to the Computer4. [Reading] The user chooses to open a file, the Computer sends the address wanted to the USB port5. [Reading] The USB returns the data at the location requested6. [Writing] The computer sends data to the USB port where it is place into empty space on the drive7. [Writing] The computer then requests a new version of the file and folder structureProsVery fast seek timesVery portable

ConsLimited capacityExpensive per MB when compared to Hard Disks

Memory cards[edit]Work in much the same way as a Flash drive and can often be converted into Flash Drives. They have different connectors and are generally smaller than USB Flash drives allowing for them to be used in cameras, mobile phones and game consoles.

comparison of different memory cards

DVD( "digital versatile disc"[4][5]or "digital video disc"[6]) is adigitaloptical discstorageformat, invented and developed byPhilips,Sony,Toshiba, andPanasonicin 1995. The medium can store any kind of digital data, and is widely used for software and other computer files, and for video programs watched usingDVD players. DVDs offer higher storage capacity thancompact discswhile having the same dimensions.Pre-recorded DVDs are mass-produced usingmolding machinesthat physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written or erased. Blank recordable DVD discs (DVD-RandDVD+R) can be recorded once using aDVD recorderand then function as a DVD-ROM. Rewritable DVDs (DVD-RW,DVD+RW, andDVD-RAM) can be recorded and erased many times.

Incomputingandoptical disc recording technologies, anoptical disc(OD) is a flat, usually circular disc which encodes binary data (bits) in the form ofpits(binary value of 0 or off, due to lack of reflection when read) and lands (binary value of 1 or on, due to a reflection when read) on a special material (oftenaluminium[1]) on one of its flat surfaces. The encoding material sits atop a thicker substrate (usuallypolycarbonate) which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with alaseror stamping machine, and can be accessed when the data path is illuminated with alaser diodein anoptical disc drivewhich spins the disc at speeds of about 200 to 4,000RPMor more, depending on the drive type, disc format, and the distance of the read head from the center of the disc (inner tracks are read at a higher disc speed). Most optical discs exhibit a characteristiciridescenceas a result of thediffraction gratingformed by its grooves.[2][3]This side of the disc contains the actual data and is typically coated with a transparent material, usuallylacquer. The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. Unlike the 3-inchfloppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.

TheZip driveis a medium-capacity removable floppydisk storagesystem that was introduced byIomegain late 1994. Originally, Zip disks launched with capacities of 100MB, but later versions increased this to first 250MB and then 750MB.The format became the most popular of thesuperfloppyproducts which filled anichein the late 1990s portable storage market. However, it was never popular enough to replace the 3.5-inchfloppy disknor could ever match the storage size available onrewritable CDsand laterrewritable DVDs.USB flash drivesultimately proved to be the better rewritable storage medium among the general public due to the near-ubiquity of USB ports on personal computers and soon after because of the far greater storage sizes offered. Zip drives fell out of favor for mass portable storage during the early 2000s

TheJaz driveis a removablehard diskstorage system sold by theIomegacompany from 1996 to 2002.Following the success of the IomegaZip drive, which stored data on removable magnetic cartridges with 100MB nominal capacity, the company developed and released the Jaz drive with 1GB capacity per removeable disk, increased to 2GB in 1998.The Jaz drive had aSCSIinterface, with both internal and external drive models. Iomega produced aJaz JetSCSI adapter PCI card for PC's. Iomega also produced a number of external adapters, including theJaz Travellerinterface that connected it to a standardparallel port, and, later, a SCSI-USB adapter and SCSI-Firewire adapter. An IDE version of the drive was planned, but never released.Disadvantages of Jaz drives Limited storage capacity(1-2 GB). Jaz disks are notoriously unreliable. Jaz drives are expensive to buy and intall($250 to $500 to buy and $30 to $150 to install). Jaz disks are expensive($50 to $150 each). You need a minimum of 5 disks, preferably 9. You need multiple disks per day for backing up large amounts of data such as your entire system.

Advantages of optical disk The cost-per-bit of storage for optical disks is very low, because of their low cost and enormous storage density. The use of a single spiral track makes optical disks an ideal storage medium for reading large blocks of sequential data, such as music. Optical disk drives do not have any mechanical read/write heads to rub against or crash into the disk surface. This makes optical disks a more reliable storage medium than magnetic tapes or magnetic disks. Optical disks have a data storage life in excess of 30 years. This makes them a better storage medium for data archiving as compared to magnetic tapes or magnetic disks. Since data once stored on an optical disk becomes permanent, the danger of stored data getting inadvertently erased/overwritten is not there with optical disks. Due to their compact size and light weight, optical disks are easy to handle, store, and port from one place to another. Music CDs can be played on a computer having a CD-ROM drive along with a sound board and speakers. This allows computer systems to be also used as music systems, whenever desired.Limitations

1. It is a read-only(permanent) storage medium. Dataonce recorded, cannot be erased, and hence, the optical disks cannot be reused.2. The data access speed for optical disks is slower than magnetic disks.3. Optical disks require a more complicated drive mechanism than magnetic disks.