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What possibly happen on your hard drive? (from www.tech-faq.com)

Hard drives, like all mechanical devices, eventually break down.
Common hard drive problems include:

User mishandlings
Software errors
Hard drive electronics failures
Hard drive arm failures
Hard drive platter failures

User Errors

User errors include accidentally deleting or overwriting files and accidentally formatting a or deleting a partition.
User errors can sometimes be resolved through the use of off-the-shelf data recovery software.

Software Errors

Software errors usually result from software writing data to the wrong part of the disk.
Errors cause by software are generally much more difficult for off-the-shelf data recovery software to correct. This task may require the services of a data recovery specialist.

Hard Drive Electronics Failures

When a hard drive physically fails, sometimes the mechanical parts of the hard drive remain undamaged. This can happen, for example, if the hard drive is subject to a power surge or a discharge of static electricity.

In these cases, it is usually possible to take the mechanical parts out of the hard drive assembly and place them into another identical hard drive unit. This should be done in a clean-room environment, to prevent dust from damaging the hard drive.

Hard Drive Arm Failures

Hard drive arm failures are very common. When the hard drive arm fails, there is a very good chance that it will damage the hard drive platters.

When you hear the clicking noises from your hard drive which signal a hard drive arm failure, back up all necessary data immediately and power the system down as soon as possible.

If the hard drive platters have not been damaged, a data recovery specialist may still be able to recover data from a hard drive with a damaged hard drive arm.

Hard Drive Platter Failures

No drive platter is manufactured perfectly. There will always be some bad spots on the platter surface. Modern hard drives automatically mark those bad spots as unusable and do not store data there.

Sometimes bad spots will develop during the life of the hard drive. The hard drive will mark that spot bad and attempt to move the data to a good spot on the hard drive. This may, or may not, result in the loss of some data.

Serious hard drive platter failures can occur as a result of hard drive arm failures. In these cases, the hard drive platters are being scratched. Your data is being scratched right off the surface of the platters! You can often hear this damage occuring. These failures are very expensive or impossible to recover.

Hard Disk Basics
Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.

At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques described in How Tape Recorders Work. Hard disks and cassette tapes also share the major benefits of magnetic storage -- the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.

Let's look at the big differences between cassette tapes and hard disks:
The magnetic recording material on a cassette tape is coated onto a thin plastic strip. In a hard disk, the magnetic recording material is layered onto a high-precision aluminum or glass disk. The hard-disk platter is then polished to mirror-type smoothness.
With a tape, you have to fast-forward or reverse to get to any particular point on the tape. This can take several minutes with a long tape. On a hard disk, you can move to any point on the surface of the disk almost instantly.
In a cassette-tape deck, the read/write head touches the tape directly. In a hard disk, the read/write head "flies" over the disk, never actually touching it.
The tape in a cassette-tape deck moves over the head at about 2 inches (about 5.08 cm) per second. A hard-disk platter can spin underneath its head at speeds up to 3,000 inches per second (about 170 mph or 272 kph)!
The information on a hard disk is stored in extremely small magnetic domains compared to a cassette tape's. The size of these domains is made possible by the precision of the platter and the speed of the medium.
Because of these differences, a modern hard disk is able to store an amazing amount of information in a small space. A hard disk can also access any of its information in a fraction of a second.

A typical desktop machine will have a hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image. No matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.

There are two ways to measure the performance of a hard disk:
Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common.
Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.
The other important parameter is the capacity of the drive, which is the number of bytes it can hold.