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
Software errors usually result from software writing data to the wrong part of
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
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 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
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
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
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.
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