It may sound unbelievable, but the deleted data is still there on your hard disk and it can be recovered. That's why it’s so important to wipe a startup disk before selling a computer, because others can access your personal data and ultimately raise concerns over your personal privacy.
As you probably know by now, there is a difference between deleting and wiping a file. While the result is the same, these two actions – affected also by the type of storage drive type – essentially control how possible file recovery will be.
In order to understand why this is the case you need to have an understanding of how HDDs work. The storage device uses magnetism to store vast amounts of data and is made of a circular magnetic disk called a platter. This is divided into concentric, circular paths called tracks, which are themselves divided into sectors. It is on here that the data is written.
How Data Recovery Works
Data management of any operating system – Windows, macOS, or any other – are all controlled by the file system, typically NTFS, FAT, or Apple’s APFS. The file system maps all data stored in these HDD sectors, knows which sectors are still unallocated, and if new data flows in it instructs the storage disk to save the data into these unused sectors.
Now, imagine that the drive is a book: the files are stored on pages of the book while the file system is the index that tracks of the location of the stored files. When a user deletes a ‘page’, its reference in the index is removed, the catch being that the information on the page is still there. On the drive this means the sector has been marked as available for overwriting but, crucially, hasn’t yet been overwritten. Overwriting data isn’t a problem for HDDs; it is normal for the drive to overwrite old data if the drive has been in use for a long time.
In other words, it is possible to recover deleted data from an HDD because data recovery software will map the drive, identify the file signatures, and be able to reindex the lost file(s). However, as times goes by, recovering a deleted file will become a harder task as the ‘page’ containing the file could get overwritten.
The same applies if the HDD is wiped. There are various ways to “secure delete” drives, but the main idea behind wiping a hard drive is to make all the data unrecoverable. If a drive hasn't been securely deleted, then it is still possible to recover at least traces of old data.
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