A common misconception is that when deleting files they are completely removed from the storage device. Data often remains on the storage media for some time after it has been deleted. Typically, files that are moved to the Recycle Bin (PCs) or Trash Can (Macs) stay in those folders until the user empties them, even then the files can still be retrieved with the right tools and knowledge.
When you delete a file from your PC, the only thing that disappears is the bit of information the operating system uses to point to that file on the hard drive. This information is used by the operating system to build a file tree structure, after this is deleted the files become invisible to the operating system but they still remain exactly where they were. The operating System just doesn't know where to look for it anymore.
The only way to completely erase data is to overwrite the section of the hard drive containing the data with new information. Over time the operating system will naturally overwrite files that have no pointers in the directory tree structure, so the more time that has passed since you deleted something, the more likely it has been overwritten. There is also "File Erasing" software products that simply overwrite empty sections of the hard drive with random strings of data.
Even with all of this, there is still no guarantee that the data will be unrecoverable. Very rarely even after every effort has been made to remove or delete data from a storage media, it could still be recoverable by someone with the right knowledge, dedication and tools. The only way to truly be sure that any data on a storage device is unrecoverable is to destory it. When the military disposes of a hard drive containing the nuclear launch codes they don't just wipe it and throw it aside. It's a much more extreme option but they will destory it just to be sure, perhaps even melting it or grinding it to dust, for magnetic hard drives you can simply wipe a strong magnet across it.
Most people however, shouldn't be destroying hard drives. It's a waste of a still functioning, usable piece of hardware. Though, if you're a business and the data contains sensitive customer information, you may want to ensure the data is completely gone rather than risking having it fall into the wrong hands.
Our statistics for last year
869 pieces storage media were received for diagnostic evaluation.
782 of these hard drives were successfully recovered.
90% data recovery success rate for hard drives.
14,322 spare parts catalogued and stored in our hard drive library.
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