Storage Devices


Definition of Storage Devices


A storage device is a device that preserves information. The information and instructions are kept in the storage device. It retains data for later. A storage device enables the computer to retrieve information directly without having to scan a series of records.

Electronic data storage is storage which requires electrical power to store and retrieve the data. Electronic data storage is easier to revise and may be more cost effective than alternative methods due to smaller physical space requirements and the ease of rewriting data on the same medium. Devices that are not used exclusively for recording and devices that are intermediate in the storage/retrieving process are not usually considered storage devices.

There are many different types of storage devices. The most popular is the hard drive. The hard drive stores everything in operating systems, applications, files, and folders for users of the computers. Even though the hard drive is the most popular there are many different types. There is flash memory, tape, disk, floppy disk, blu-ray, DVD, CD-R, and CD-RW removable hard disk, micro drive, compact flash and SD cards, zip drives, and RAM.

There is a primary memory and a secondary memory. The primary memory is the unstable memory and the secondary memory is the stable memory. The unstable memory is the kind of the memory that is erasable and the stable memory is the one where in the contents cannot be erased. There is also internal and external storage. They are one of the most important components of the computer. Data storage device is a device used for storing and recording data. A storage device may process information, hold information, or sometimes both.


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History of Computer Storage


Before the 20th Century

1725-Punch Cards


The earliest known storage device was first invented in 1725 by Basile Bouchon to work in the textile industry. This mechanism consisted of pegged cylinders that were controlled by perforated paper tape. The tape was a punctured paper loop that stored patterns, but the first real useful storage device is dated back to 1884. This was created by Herman Hollerith. His invention, the Punch Card, was used for nearly one hundred years. Typically, each card had eighty columns, and they could not store very much information. There main use was to store settings for different machines, not data. To protect the patents on these cards from competitors, the company IBM locked them up. Soon though, a man named Remington Rand, who owned a well-created line of machinery, eluded Hollerith’s patents and used ninety column cards with round holes. Each punch card used a binary character code that was extremely efficient. This invention was used until about the mid-1970’s.

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1846-Punched Tape


Punch Tape was created by Alexander Bain in 1846. Originally, it was used for helping with mechanized looms in the textile industry. It could be used for data input and output. It consists of a long strip of paper with holes punched in it to store date. Each stripe first consisted of five rows but as the years went on the tapes started to hold six, seven, and eight rows. One character is represented by each row on the tape. The first standard character encoding known was Baudot, and it held five characters. Some later known encoding characters include Teletypesetter, Fieldata, and Flexowriter. They each had six holes. The most current known encoding is called ASCII and it is a seven-character code Punch Tape was widely used in the 20th century for teleprinter communication and minicomputer storage.

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1900's-1950's


1928-Magnetic Tape


The magnetic tape was developed by Fritz Pfleumer in 1928 but was not used to record data until 1951. The recording medium was a 1/2 inch wide thin band of nickel-plated bronze. The recording density was 128 characters per inch on eight tracks at a about the speed of 100 i/s, yielding a data rate of 12,800 characters per second.


1932-Magnetic Drum


The magnetic drum was invented by G. Taushek. How it worked was a ferromagnetic layer on the outside of a metal drum read and write heads were mounted at a distance of some micrometers that produced an electromagnetic pulse. This pulse could be stored by changing the magnetic orientation of the ferro magnetic particle the write head passed over. Therefore binary values of 0 or 1 are recorded by generating electric pulses while the drum is rotating. The magnetic field of some of the particles suspended is polarized along a "track". Reading back the recording is done by detecting which particle was polarized and which was not. The capacity of a drum about 500.0 bit.


1946- Williams-Kilburn tube


The Williams-Kilburn tube was the first random access computer memory, and it stored data through using electrostatic cathode-ray display tubes as digital stores. The advantage of the Williams tube memory was that it allowed fast access of memory.


1949- Delay Line Memory


The basic concept of a delay-line memory consists of inserting an information pattern into a path which contains delay. If the end of the delay path is connected back to the beginning through amplifying and timing circuits, a closed loop is formed allowing for recirculation of the information pattern. The delay medium should slow the propagation rate of the information sufficiently so that the size of the storage equipment for a large number of pulses is more understandable. A hard disk uses hard and rigid rotating platters. It stores and retrieves digital data from a planar magnetic surface. Information is written to the disk by transmitting an electromagnetic flux through an antenna or write head that is very close to a magnetic material, which in turn changes its polarization due to the flux. The information can be read back in a reverse manner, as the magnetic fields cause electrical change in the coil or read head that passes over it.

1960's and 1970's

-The Evolution of Hard Drives and Disk-to-Disk Backup

In 1956, IBM introduced the first hard drive, which was IBM 305 RAMAC. Over the years HDD technology has been improved quickly. Since 1983, hard disk drives have become a standard component for most personal computers. For example, in 1982, Hitachi shipped the first drive which held more than one gigabyte of disk storage. One more important event was the introduction of the RAID, or redundant array of inexpensive disks, technology in early 1990s. This data storage scheme uses many hard drives to share data through them. Storing data on hard disks became a useful and helpful solution due to these improvements. The amount held several hundreds of gigabyte priced at about $300 is today’s reality.

In the 1960’s and 1970’s, hard drives were not suitable for backups, because of their high price, large size, and low storage. Already in the mid 1980’s, hard disk drives could already be well thought-out for making backups. In the early 1990’s, hard disk drives became an alternative for tape reinforcements. Today, the "battle" between tape and disk backup still rages on.

-Floppy Disks and their Contribution to Backup

The first floppy disk was introduced in 1969. It was an eight inch, read only disk that could store eighty kilobytes of data. In 1973, a similar floppy disk that was the same size, could store 256 kilobytes of information. It was also rewritable. Since then, the development of the smaller floppy disks and higher data storage has stayed the same. In the late 1990’s, it was an easy process to store 250 megabytes of storage on а three inch floppy disk.

Floppy disks were believed to be a revolutionary media for transporting data from one computer to another. They could not store as much data as hard disks, but were much cheaper and more flexible. They became very widespread. This trend affected the reinforcement sphere. Since 1973, after eight inch disk, the SSSD became common. It was being used to move low amounts of storage around. Floppy disks began to be widely used for backup purposes. Floppy disk backup was not as wide-ranging as tape backup. But as these disks were rather cheap and very handy, they quickly became one of the most prevalent backup media among home users and small companies.

-CD-R/RW and DVD

Though, the 3.5-inch floppy disk had been a benefit to home users and small businesses who needed backups, they had relatively low capacity. This problem had been solved with introduction of the next generation in storage media: CD-Recordable (CD-R) and CD-Rewritable (CD-RW) drives. The Compact Disc, first invented by Philips and Sony in 1979, reached the market in late 1982 in Asia and early the following year appeared in other markets.

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Cassette Tapes


With advances in magnetic tape data storage, cassette tapes eventually became common data storage options in the 70s and 80s. Cassette tapes stored up to 660k per side, boasting more storage than any removable media to date.

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1980's

Prior to the 1980’s, storage devices were still magnetic strips and punch cards. Advancements in disk technology, though, quickly outpaced the speed and efficiency of tape systems—resulting in the obsolescence of tape for most applications.
Hard disk and floppy drives both came out in the 80’s. Personal Computers relied solely on the use of these for a long time until the 90’s. More than 66 million Winchester hard drives and floppy disks were sold per year by 1990.
As computer memory storage device manufacturers entered the 1990s, new storage technology was beginning to gain widespread attention by the industry and consumers. Optical memory, which had been viewed essentially as an experimental or specialty technology during the late 1980s, was beginning to establish itself in mainstream business and consumer markets. There was also an increasing interest in semiconductor memory.


1990's


In the 1990’s, the internet we know today was formed. The release of the internet triggered a massive need for more memory. In the 80’s, floppy disks and CD-ROM’s were where most data was stored, not counting computers’ built in memory of course. The 90’s introduced new forms of data storage which include—

Magneto-Optical Disk

The Magneto-Optical Disk (or MOD for short) was a revolutionary idea. It allowed users to rewrite the CD over and over! The only problem was that to use these disks, you had to have a Magneto-Optical Drive built into your computer. Today the disks are outdated and are quite difficult to find. The modern version of the MOD is the CD-RW.

MiniDisc

A MiniDisc is a disc-based data storage device or storing any kind of data, usually audio. It was announced by Sony in 1991 and was designed to replace analogue tapes, and to take the music industry into the future. To this day, it marks a very important stage in the music storage industry.

CompactFlash R

A CompactFlash (orCF) was one of the first mass produced Nano-technological devices in history. At about an inch and a half, this is one small piece of technology. even though it only stored around 64 MB, this was a big deal. With the ability to make devices as small as the CF, engineers started furthering their research into nano-technology. THroughout the rest of the 1990's, this CompactFlash was followed by the "Multimedia Card" and the "Memory Stick".

DVD


A DVD wasn’t a huge achievement in the field of computer engineering, but it was a big deal. Basically, a DVD is essentially a bigger, faster CD that can store movies and video games. This product helped the Video Game and Movie Industries to bloom.


CD-RW

As I have already mentioned the MOD, there is no need to delve into the subject deeply. It is basically just a MOD that is capable of storing more and being rewritten more times.

Types of Storage Devices and their Current Uses



Blu-ray


A Blu-ray disc is a high density optical disc. The Blu-ray disc can be used as a storage device. It can also be used for high definition audio and high definition video. This disc is the newest disc out at this time. The reason it is so popular it because it projects such clear images and such good audio. It can do this because it uses a laser to write and read data. Regular CD’s and DVD’s use an infra-red laser, while the Blu-ray disc uses a high frequency blue laser. The advantage of using the blue laser is because it has a shorter wavelength than a red laser. This results in even greater precision. With more precision, you are able to pack more data on a disc so using this laser is very efficient. Some people may think that the reason this disc can store more data is because it is bigger. that is a false statement! The Blu-ray disc is the exact same size as a CD and DVD. The blu-ray is mostly used for the PS3 or for watching movies on a Blu-ray player. You can also watch DVD’s with the PS3 because it is installed with a Blu-ray player. Right now, the only uses for the Blu-ray disc is in the PS3 and the Blu-ray player, but they are exploring for new uses of this breakthrough in technology.

RAM


RAM is your random access memory; this is the equivalent to our short term memory. The more RAM your computer has the more tasks you can complete at once without your computer bogging down. You can't really store anything on your RAM for much longer than you're doing a certain task. You can get up to 8GB total of RAM in your computer currently. RAM is an internal part of your computer so most people don't end up touching it at any point. That also means it's not portable, it stays with your computer. Random-access memory is a form of Today, it takes the form of integrated circuits that allow stored data to be accessed in any order. The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.
By contrast, storage devices such as magnetic discs and optical discs rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than data transfer, and the retrieval time varies based on the physical location of the next item.

The word RAM is often associated with volatile types of memory (such as DRAM memory molecules, where the information is lost after the power is switched off. Many other types of memory are RAM, too, including most types of read only memory and flash memory called NOR-Flash

Your computer probably uses both static RAM and dynamic RAM at the same time, but it uses them for different reasons because of the cost difference between the two types. If you understand how dynamic RAM and static RAM chips work inside, it is easy to see why the cost difference is there,­ and you can also understand the names.

Dynamic RAM is the most common type of memory in use today. Inside a dynamic RAM chip, each memory cell holds one bit of information and is made up of two parts: a transistor and a capacitor. These are, of course, extremely small transistors and capacitors so that millions of them can fit on a single memory chip. The capacitor holds the bit of information -- a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.

This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.
Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory. A flip-flop for a memory cell takes 4 or 6 transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes a lot more space on a chip than a dynamic memory cell. Therefore you get less memory per chip, and that makes static RAM a lot more expensive.
So static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.

Hard Drive


A hard drive, also known as an HDD, is a device that stores massive amounts of information and retains information stored even when not powered.
An HDD stores information by writing the information on plates called platters. These platters are made of nonmagnetic material, such as an alloy, and are coated with a thin layer of magnetic material. When information is written on platters, they spin at high speeds (up to 15,000 RPMs) while read and write heads attached to actuator arms, located nanometers from the platters, move in an arc to modify the magnetization of the magnetic coat on the entire surface. This writes a series of ones (1) and zeroes (0) which are called the binary language. Binary language tells the computer what to present on the monitor when the information is accessed later.


There are two types of hard drives :
Internal Hard Drives: These are HDDs that are compact enough to be placed inside a computer. All modern computers along with other devices such as cell phones, iPods, and video game consoles contain an internal hard drive that have the ability to hold up to two terabytes (two trillion bytes) of information.

External Hard Drives: These were the humble beginnings of hard drives in which they were so huge, that they could not fit inside a co mputer. So, they were placed in a protective layer and attached by means of a wire such as a USB port. Today, external hard drives are still sold to hold inform ation safely in case a computer crashes, but are now incredibly small.

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Today, especially in laptops and other mobile devices, shock resistant hard drives are very important. This makes sure that the read and write heads do not touch the platters if the device is dropped. If the heads hit the platters, all the memory will be erased because the magnetization will be messed up. Shock resistant hard drives help avoid this.

HDDs today also use Error Correcting Codes, or ECCs. These codes store extra bits of information for each bit added. The amount of information is determined by a mathematical formula. By doing this, it alerts the computer on errors that need to be fixed and also allows a high recording density which means the HDD has a larger storage capacity.

Memory Cards


A memory card is solid-state electronic data storage device capable of storing digital contents. They are used in almost every type of small electronic device. They offer high re-record-ability, power-free storage, they are small in size, and easy to use. They are used to save data in digital cameras, digital camcorders, handheld computers, PDA’s, media players, mobile phones, GPS receivers, and video game consoles.
There are three different speeds of memory cards with completely different uses. The first speed is Ultra II which has a minimum read speed of 15 megabytes per second. It is used for devices like basic cameras that do not require a fast data transfer speed. Extreme III, the second data transfer speed, has a maximum speed of 30 megabytes per second which is for devices that require a medium speed of data transfer. It is used for devices like high-quality cameras. Extreme IV has a maximum transfer of up to 45 megabytes per second. It is used for recording videos on cameras and high-graphic video games. All types and brands of memory cards can be classified under these three categories.


USB flash drives



A USB flash Drive is a data storage device integrated with a USB interface, which is the area that plugs into different devices to create communication and data transfer between the two. USB drives are a type of EEPROM, which stands for Electrically Erasable Programmable Read-Only Memory. They are a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed.
USB flash drives are used for quick data transfer and are easily removed after transfer and saving small amounts of data. They are used with PDA’s, laptop computers, digital audio players, digital cameras, and cell phones. They are small and portable. Data on USB flash drives can be transferred to other devices through connecting the USB interface to the other desired device.



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Common Storage Devices

Floppy Disk


A floppy disk is a flexible square plastic disk that is coated with magnetic material and covered by a protective jacket. They are also known as a diskette. Floppy disks have a silver or black sliding piece going across the top. They are used primarily for computers to store data magnetically. These devices are capable of holding about 144 million bytes.

Zip Disk


Zip disks look like floppy disks, but they are thicker. They come in 100MB, 250MB and 750MB varieties. These devices are a floppy-like technology that use design concepts from hard disks. The drive is bundled with software that can catalog the disks and lock the files for security. When Zip disks were introduced in 1995, they became popular very quickly.

CD-RW


CD-RW discs look like other CD media, but with close inspection, they have a more polished surface with a very dark blue-gray cast. Similar to a hard disk, files can be added and deleted, and the media must be formatted before use. However, unlike a hard disk platter, which can be rewritten millions of times, CD-RW’s have a maximum limit of 1,000 rewrites. A CD + RW disc holds about 650 MB.

CD+R


CD-R discs are blank CDs that can record data written by a CD burner. The word "recordable" is used because CD-R’s are often used to record audio, which can be played back by most CD players. However, many other kinds of data can also be written to a CD-R, so the discs are also referred to as "writable CDs." The data burned onto a CD-R disc is permanent, meaning it can not be altered or erased like the data on a hard drive. CD-RW’s can be erased and completely re-recorded.

DVD-R Disc


These discs hold the space of about 4.7 GB and are used to record movies.

Uncommon Storage Devices


This is a disk drive in which a plastic or metal case surrounds the hard drive. It can be inserted and removed just like a floppy disk. It holds about 2 GB of data.

Internet Hard Drive


The Internet Hard Drive is a service on the global Internet that provides storage space to computer users. This service offers about 25 MB of space, but it could provide more, depending on the service type.

Flash Drive


Flash drives are also known as USB drives. A flash drive is a small, stick-like portable storage device which, unlike a hard drive or optical drive, has no moving parts. Most flash drives connect to the PC via a built-in USB port. Their storage capacity ranges from as small as 16MB to as much as 64GB and more. Flash drives can be written and rewritten to an almost unlimited number of times, much like a standard hard drive. This is a credit-card sized removable module for portable computers standardized by PCMCIA (another name for a PC card). It fits into a PC card slot, usually found on a notebook computer. This card simply adds storage to most notebooks.

Smart Card


A smart card is an ATM-sized card containing a computer chip which enables the holder to purchase goods and services, enter restricted areas, access medical, financial, or other records, or perform other operations requiring data stored on the chip. When inserted into a smart card reader, it can read and update data for you.

Storage Tape


This one is a magnetically coated ribbon of plastic, capable of storing large amounts of data at a very low cost. Usually, storage tapes are a little bigger than audio tapes. Older computers used tape and tape drives, but even today, some people still back their systems up with storage tape. These tapes hold between 20 GB to about 110 GB of data. An external tape drive can be purchased separately as well, but those are even harder to find.

Miniature Mobile Storage Media


This refers to flash memory cards, USB drives and tiny hard disks. It is used mostly with handheld computers and digital cameras.

Memory Stick


A memory stick is a type of flash memory developed by Sony; it is a rectangular shaped disk. It is used to store data for digital cameras, camcorders, notebook computers, and other kinds of electronics. Because a memory stick is a proprietary Sony product, it is used by nearly all of Sony's products that use flash media. Unfortunately, this also means memory stick cards are incompatible with most products not developed by Sony. Memory sticks are capable of holding approximately 128 MB of information.

Microdrive


A microdrive is an ultra-miniature hard disk from Hitachi Global Systems. The microdrive was developed and introduced by IBM in 1998 and acquired by Hitachi in 2002. The microdrive uses a single platter the size of an American quarter that holds up to 8GB. Using one or two GMR heads, the entire mechanism is built into a Type II Compact Flash form factor.

SmartMedia Disc


This is an ultra-compact flash memory format developed by Toshiba and introduced in 1995. About the size of Compact Flash, but as thin as a credit card, SmartMedia cards were popular storage devices for digital cameras, but capacities never reached beyond 128MB. Available in 3.3 and 5 volt variations, SmartMedia cards require no assembly in manufacture as they are actually flash memory chips in a unique chip package. These cards can be plugged into a SmartMedia socket or into a standard Type II PC Card slot with a PC Card adapter.

Future Storage Devices


Holographic Memory Data Storage


After 30 years of research scientists think they have discovered the HDSS. (Holographic data storage system) This type of storage device will be faster and more efficient. HDSS will be able to be mass produced which means more HDSS will be shipped faster and be available more often. What is Holographic memory? It is a hologram that has a photographic record of the content. HDSS uses laser beams and light to read and store the data. HDSS will be able to store more information than a regular Storage device. It will also be smaller and more convenient for use. Most computer hard drives only hold 10 to 40 GB of data, however HDSS will hold a lot more. These are the basic concepts needed in a HDSS:
• Blue-green argon laser
• Beam splitters to spilt the laser beam
• Mirrors to direct the laser beams
• LCD panel (spatial light modulator)
• Lenses to focus the laser beams
• Lithium-niobate crystal or photopolymer
• Charge-coupled device (CCD) camera

Solid State Memory



Solid State Drive (SSD) A type of computer memory that is stored within a hardware device that contains no moving parts. For example, Compact Flash memory is a solid state memory. This is much like a flash drive USB except that a flash drive is out side of a computer and you must plug it in. A SSD is inside the computer. An SSD is more convenient then a flash drive because a flash drive has moving parts such as a motor while the SSD has a flash memory so it doesn’t need to move. This results in the SSD
having:
• Less power usage
• Higher reliability
• Faster data access


Molecular Storage Memory


Molecular storage memory is data storage technology that uses Molecular species for data storage. It uses molecular species rather than circuits, magnetic, inorganic for data storage device. This device can be called the Molecular Switch. It performs several functions like charge storage, photoschromism, or changes in capacitance. A well put together molecular memory device each molecule contains some data, making it able to store a massive amount of data. In 3D optical data storage, they use larger numbers of molecules for each amount of data. The molecular memory has been developed since the early 1990s. Molecular memory can only be found in laboratories, and it is still limited even to them.

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Brands of Storage Devices:



Sandisk-


History-

Sandisk was created in Delaware in June of 1998. The company currently trades on the NASDAQ. Sandisk is the inventor and worldwide leader of NAND-based flash store cards. The company designs products and solutions in a variety of form factors using flash memory, controller and firmware technologies. They have more than 3,400 employees and are currently located in Milpitas, California.

Products and Quality-
Sandisk 16GB Express ExpressCard:
1. very high speed
2. provides super-fast downloads of large videos
3. can read and write data 2.5 times faster than PC cards
4. Features:
Super fast transfer speeds of large video films from media to computer
Approximately 2 hours recording time with 16GB memory card
Goes directly from camera to Express Card slot into the computer
5. it is priced at about $999.99
Hewlett-Packard:

HP-



History-
Two men from Stanford University by the names of Bill Hewett and Dave Packard founded HP in 1939. The first product was an audio oscillator. One of their first customers was Walt Disney. They purchased eight oscillators to help produce the movie Fantasia. They continue to make computers, printers, and storage devices. They are rated one of the top companies for these products.
Products and Quality:
HP Serial ATA/300 Internal Hard Drive:

1. 250 GB
2. 7200 RPM
3. transfers data faster than most
4. priced at about $99


Sony-



Sony is one of the world most widely known electronics company. It was founded in Japan in 1946, by Masaru Ibuka and Akio Morita. Sony has been a profitable company for more than 60 years. Sony has a long history of introducing technologies. For example in 1955, Sony introduced Japan's first transistor radio, the TR-55. They are a very dependable company and sell many different storage devices like the Sony NEC Optiarc AD-7590A 8x DVDRW Slim Drive.


Seagate-



Seagate is the world’s largest manufacturer of hard drives and storage solutions. The company was founded in 1979 and is in Scotts Valley, California. Seagate’s hard drives are used in a variety of computers, from servers, desktops, and laptops, to other consumer devices, such as digital video recorders, Sony’s PlayStation 3 and Microsoft’s Xbox and Xbox 360 video game consoles, and in portable media players and automotive navigation systems. One example of a Seagate storage device is the Cheetah.
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CORSOIR-



Corsair is a company that has been producing high-speed devices for a long time. They started off working with mainly memory modules, but have now started working in the field of storage devices. Corsair makes very dependable flash drives that range in storage space from 512 MB all the way to 128 GB. This company has the average looking flash drives, but they also have mini-flash drives and even an indestructible one. Their indestructible flash drive is anti-shock resistant, can be in water up to 200 meters deep, and comes in an aluminum case that is very durable. Majority of their flash drives are backwards compatible with USB 1.1, which means it is compatible with the older style of USB ports.


MEMOREX-



Memorex is a company that has been around for a while. Recently they began to make Blu-ray, they have one disc out right now that is a 25 GB. The individual single disk sells for $12.99, but they also offer the disks in packs of three, five, fifteen, and twenty-five in writable and rewritable. These disks have the clearest picture that is currently available on the market. They utilize a blue-violet laser technology, and have five times that storage capacity of the average DVD orsingle layer disk. The only downside to their Blu-ray disks is that they are only compatible with Blu-ray devices.



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