When you get rid of sensitive paper documents, it's a good idea to shred or burn them to help protect your privacy and prevent identity theft. Similarly, it's important to erase your personal or sensitive information from computers (desktop, laptop, or tablet) and other devices (smartphone, gaming console) before you dispose of them.
When you delete a file, it is quite often not actually deleted. If you use Windows it is simply moved to a protected system folder and is shown in the Recycle Bin. Even when you empty the Recycle Bin the file is not deleted. The area on the hard drive where the data resides is marked as free so that the operating system sees it as available space, but the data is still there. With special software, you can recover this data and view personal information.
There are many data recovery techniques that you can use to undelete or recover the deleted data, and to prevent this you must perform some additional tasks.
The most common technique is digital shredding. This process does not delete the file as such, but it does overwrite the data with a series of 0 and 1. Firstly it writes the data with all zeros, then all ones, then a random sequence of 1 and 0. Writing all ones and all zeros align the polarity of the physical data bits on the hardware, finally, a random write eliminates any chance of the data to be recovered.
There are several free programs that can perform this task, and they all function in pretty much the same way.
Hard Drive Shredding
You can also perform shredding on an entire hard drive, except that instead of erasing a file, or collection of files, it destroys the data on all sectors of all tracks on every platter of the hard drive. Again there are several free hard drive erasure tools available, and offer various levels of guarantee on the effectiveness of the process. It should be noted that this process can take considerable time on larger drives, and should not be used on SSD's as the process may damage the memory or lower the drive's life expectancy.
Physically Wipe Hard Drive
If you have data that you really do not want anyone to find (such as MI6, CIA, FBI mainframe hard drives) then the best option is to physically render the data on the hard drive useless. There are various methods you can employ to achieve this task, including dismantling the drive and scratching the platters with a screwdriver, smashing them with a sledgehammer or using a blowtorch to burn the surface and warp the platters.