Hard drives standards and purchasing.

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If you have ever taken a looksee in the hard drive market, you'd know that there are many different formats and standards that are available.  Many end-users looking to 'upgrade' their PCs by buying a new hard drive are caught up in this and land themselves unsure of what to choose.  This guide will try to make sense of the different hard drive standards and technologies available.  It is split into sections for ease of reading.

Rotational Speed
This is the speed at which the platter inside a hard drive spins, this does not affect the read and write speed of the drive as much as it does the Seek time and Rotational Delay.  (The read speed being the rate at which the hard drive can source data, and the write speed being the rate at which it can record data (usually given in 'x'MB/s).  The latency is how long it takes for the drive to perform a given action.  The Seek Time is the time it takes for the read/write head to find the required cylinder on the platter, E.g. the time it takes for the hard drive to find the certain file.  The second latency is the Rotational Delay, which is the time for the required area of the disk to rotate so that it is under the read/write head.  I.e. the read/write head is an arm that moves over the surface of the disk within the HDD, even if the head is in the right location, it still needs to wait for that area of the disk to come round (the head is solid mounted, so can only swivel over a finite portion of the disk surface, an image is included at the end of the guide for clarification).  Thus the seek time and rotational delay added are the total time it takes for the hard drive to find the required file etc.

A general rule-of-thumb is- The higher the rotational speed, the lower the rotational delay (as the disk is spinning faster).  Seek times are generally quicker with higher speed drives aswell, as manufacturers tend to put better motors in the read/write arms.  The market standard for desktop hard drives (3.5") is currently 7200RPM rotational speed.  You will only ever find new hard drives lower than that in notebook format (2.5"), where the standard is 5400RPM, and rising to 7200RPM. 

To make it simpler, these are the RPM standards that are commonly found in hard drives:

4200RPM Almost completely phased out
5400RPM Found in lower spec. new laptops and older laptops (2.5")
7200RPM Industry standard for desktops (3.5") and higher spec. laptops (2.5")
10,000RPM Only one drive available for regular enthusiasts (WDC Raptor), otherwise they only come in SCSI format
15,000RPM Only available in SCSI format

The Western Digital Raptor is currently the only 10,000RPM hard drive that is available in PATA or SATA formats for regular users.  All other drives at 10,000RPM and higher are only available in SCSI format.

Hard Drive Interfaces
There are many formats used in the hard drive industry:

PATA- Industry standard, though likely to be phased before too long, uses the widely known 'Ribbon cables' to connect to other devices
    ATA-33, ATA-66, ATA-100, ATA-133 are the formats used in the PATA interface, ATA-100 being the most common, as many drives can easily     push 66MB/s (ATA-66), but none can touch 133 or even 100MB/s).
SATA- Replacement for PATA, quickly becoming the industry standard, has much smaller cables than PATA interface and better configuration
    SATA-150 has a maximum bandwidth of 150MB/s (no single hard drive in existence can transfer data at this rate, simply because the physical               technologies aren't refined enough (E.g. rotational delay, bearings etc. etc.).
    SATA-300 has a maximum bandwidth of 300MB/s, the comment above applies here too.
SCSI- Has a long history, but only useful for server configurations now that other standards like PATA and SATA are available.  It is used in servers as     it is the most convenient standard when using large, RAID arrays (a RAID array is when multiple hard drives are connected to a controller and             configured as one drive, an entire separate guide can easily be written on this subject).
    There are many different formats and even physical interfaces used in the SCSI interface.

For the regular user, only SATA and PATA drives are of interest, as brand new SCSI drives usually only come in 10,000RPM and 15,000RPM formats, and also require a separate SCSI controller (PCI card), and are thus very expensive to get running.  Note: SATA and PATA both use the IDE standard (Further note: PATA is often mislabelled by vendors as simply IDE, this is a huge contradiction as SATA also uses the IDE format, however if you see a hard drive being advertised as just 'ide hard drive', it will most likely be PATA).

As mentioned above, the speed ratings for each standard are often misleading, for example, SATA-150 CAN transfer data at 150MB/s, but no hard drive in existence can go anywhere near this speed, because it would require extremely refined motors and workings that currently aren't available.

If you are buying a new hard drive, try to go for SATA as it is much more convenient (doesn't use the bulky ribbon cables that the PATA interface does).

On a hard drive, a cache is a temporary store where recently accessed files are stored.  A cache allows quicker loading of files and program that are used regularly by a user.  You may often notice that when you first boot into your Operating system (E.g. windows, linux etc.), a certain program may take a while to load first time, afterwards however it is much, much quicker, this is likely because it is still in your hard drives cache.

Most 7200RPM drives come with at least 4MB of cache, slightly higher spec. drives come with at least 8MB, as the capacity of the drive increases, generally so too does the amount of cache, most drives above 200GB come with 16MB of cache, instead of the 8MB that accomponies most around the 80-200GB mark.  If you are buying a hard drive for general use look for at least 8MB of cache, anything under is usually not worth your money.

2.5" and 3.5"
These terms have both been used so far in this guide, a 2.5" Hard drive is one that is designed for use in laptops, do to the confined space, however they can be used just like a normal hard drive, albeit they are usually considerably worse performance.

A 3.5" drive is your normal hard drive size, all non-laptop hard drives come in this size, even the 15,000RPM SCSI 'screamers'.

The capacity of a hard drive is how much data it can store, capacity usually comes in several standard sizes:

40GB  Lower than industry standard, 40GB and 60GB drives aren't very popular as they are relatively very small
60GB  Lower than industry standard (for factory PCs)
80GB  Former factory PC standard,  usually have 8MB of cache
120GB Becoming the new factory PC standard, drives between this size and 200GB are the most popular among enthusiasts as they can easily achieve             high capacity when configured in RAID arrays, usually have 8MB cache
160GB Usually have 8MB cache
200GB Usually have 8MB/16MB cache
250GB Usually have 8MB/16MB cache
300GB Usually have 16MB cache
400GB Becoming more and more popular for RAID/SCSI arrays in file servers. 16MB cache
500GB Only a few drives of this size are available, and are very expensive compared to smaller version (not just total cost, but $/MB aswell (I.e.  when divided out, how many MB/GB etc. you get per $). 16MB cache

Solid State Hard Drives
Solid state hard drives are drives which use RAM to store data.  The usually comprise of some sort of controller that separately bought RAM is mounted in, the controller is then seen as a normal hard drive in the user's OS.  The most popular solid-state hard drive on the market currently is the Gigabyte i-RAM, which can take upto 4GB of DDR400, and can run at an average read rate of approx. 130MB/s.  Because they use RAM to store data, solid-state disks can operate at extraordinary rates, and have a straight read/write rate accross the entire space, as there are no imperfects or different rotational delay (there are no moving parts!).

open your hard drive casing unless you intend to break/mod. it.  Opening the casing allows air to enter the chamber.  Hard drives are manufactured in rooms that are atmospherically controlled, to ensure no dust or stray particles enter the process.  The chamber inside a hard drive is completely sealed, and has a factory defined air pressure for optimum performance, thus any leaks can cause damage and allow fine particles to enter.

The gap between the read/write head and the hard drive platter is small enough that a single particle of dust could wedge itself between it, causing severe damage to the drive.  If you have a drive that has been physically damaged, E.g. broken motor, or the cover has been removed, you can pretty much give up all hope of ever getting any of the data back.   Because they are made to such strict tollerances, it's not something you can fix in the back shed.

General Advice on purchasing
When purchasing a hard drive, on eBay, in a shop, or through an internet vendor, the manufacturer and model will usually be included, information on that particular model can always be found on the manufacturers website if it isn't already included by the vendor.  Like most purchases, it isn't wise to buy if you aren't sure of the item specifics.

This is an image of a hard drive with the cover removed:

The circular disc taking up most of the area is the platter, on which the data is stored through electromagetic orientation of cylinders (virtual segments of the platter).  The triangular bridge sticking out into the middle of the platter is the read/write arm, it can easily be seen why it relies on the platter to rotate into position.  The bright metalic objects top-left of the arm base are the motors and components that drive it out, these operate at incredible speeds (of ten 25Hz or more).  The middle of the platter shows the locking mechanism that secures it to the axle of the spindle drive (motor).

This is the first guide I have attempted so there is inevitably going to be lots of comments that aren't right on the mark etc.  I have also been very brief in my explanations, and left out most things that do not concern the average user.  I hope this guide may be of use to someone, and thanks for reading

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