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It's inevitable. At some point in your life, you're going to have data on a hard drive which has either become inaccessible or which has died. Now while we can always prepare for a hard drive failing, the discovery that your data is in jeopardy is usually accompanied by a sinking feeling. What is on the drive? How valuable is it? And will I ever be able to get it back?
Don't give up hope. Retrieving data from a hard drive is not impossible, and in most cases a hard drive which has died will still contain salvageable data. Complete loss of data usually only occurs in situations where the drive head for some reason comes into contact with the disks and physically damages them. This most often happens if a hard drive which is in use is dropped, knocked over, or otherwise jolted. As long as your hard drive hasn't been knocked about the head, it's data should still be recoverable.
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The Disappearing Drive
The first step towards recovering lost data is figuring out why the hard drive is no longer working. There are many reasons a hard drive could disappear or become non-responsive, and most of them don't mean that the hard drive itself has failed. A computer, with its army of drivers, can be finicky to changes.
In order to determine if your hard drive has failed, you'll want to test the hard drive in another computer. This means taking the hard drive out of your current PC and placing it into another one. This is easier than you might think, because hard drives are plug-and-play devices, so long as you're dealing with a modern SATA drive. Once you have installed the hard drive into the new machine, Windows should detect it and make it visible. If you're moving a drive between PCs that are using different versions of Windows, then the test PC may very well ask to reformat or modify data on the hard drive you're trying to save. Be sure to pay attention to all prompts, so that you do not accidentally reformat the drive you're trying to retrieve data from.
Of course, it is likely that you don't have a second computer on which to test a hard drive. If that is the case, then you'll want to buy a SATA to USB adapter. These can be found at your local Radioshack or local PC repair shop, although you'll save a lot of money by buying one online. Once you have the adapter, use it to plug your hard drive into a USB port on your PC. Doing this should cause the hard drive to appear so long as it is still alive.
If either method causes your hard drive to appear, go ahead and grab your most important files before placing the hard drive back into your PC to search for the cause of the drive's disappearance.
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The worst case scenario for data recovery is a mechanically dead hard drive. Recovering data from a hard drive suffering a mechanical failure is difficult because components inside the hard drive itself have ceased to function.
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The Illogical Drive
Drive failures boil down to two basic types - logical and mechanical. Logical failures have to do with the failure of the drive's ability to interpret its own file system or communication with a PC. Logical failures rarely effect the data on the hard drive, and the physical disks inside the hard drive should not be damaged in any way. This means that retrieving your data should be possible on your own, although there are two methods for recovery. The one which I would suggest for the average user is to invest in software specifically made to recover data from a hard drive suffering a logical failure. I have had luck using Ontrack Data Recovery's data recovery software in the past, but there is a wealth of other software meant for the same purpose. The advantage of this method is that it is simple. Data Recovery software is easy to use, and if the recovery is successful then the process of data recovery should be painless. On the other hand, data recovery software isn't cheap, and there is no guarantee that the software will work.
If the data recovery software does not work, and/or you're an advanced user who wants to save a few bucks, then it is possible to defeat the problem by replacing the logic board on the hard drive. If you're unsure how you would do this, then I wouldn't suggest trying it. Although it often works, the process requires knowledge of the hard drive's layout, a replacement logic board and (in some cases) a soldering gun.
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The Broken Drive
While logic failures occur when the hard drive's logic board becomes confused and unable to navigate to its own file system, mechanical failures occur when physical components in the hard drive have stopped functioning correctly. Mechanical hard drives have numerous moving parts which can become damaged over time, and failure of the motor or the read/write head are serious issues. A mechanical drive sometimes announces its failure by making clicking noises, but this is not a 100% accurate method of determining if a failure is mechanical in nature. For example, some lower-price Maxtor drives sound like they're failing right out of the box, but are in fact working as they should.
A mechanical failure is the culprit, then you could be in trouble. Mechanical failures are the worst type of failure and usually require the assistance of a data recovery professional. There are numerous rumors on the Internet concerning how to recover a hard drive. Some suggest that you can fix your hard drive by putting it in your freezer, while others say you should tap your hard drive hard against a table. In theory, these fixes could work. But they also run a high chance of further damaging your drive, and I do not recommend them.
If you do need a data recovery professional, the first number to call is the warranty department of your hard drive manufacturer. Explain to them your problem and ask that they cover data recovery. Many hard drive makers have their own data recovery services or work closely with third party data recovery companies, and you may be able to convince them to cover the cost of recovering the lost data. If this option is not available, then you should prepare to fork out some cash. Data recovery will cost at least several hundred dollars, and severely damaged drives can run your bill up to and over a grand. Their expense is justified by their talent, as data recovery professionals can work miracles, even if their price makes them a last resort.
To understand how data recovery works and its limitations read An Introduction to Data Recovery by Matt Casperson.
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Images are from press materials of hard drive manufacturers Western Digital and Seagate.