Why Deleting or Formatting the Drive is Not Real Protection
You might know that simply deleting a file is not enough. When you delete a file, the operating system just removes references from the file allocation table instead of actually removing the data. It's sort of like ripping the cover off a book but leaving the pages intact. Anyone with undelete software can access this data and use it for personal gains.
Formatting is almost the same. It offers somewhat greater security as it formats the file allocation table but even then, the data is recoverable. Computer experts with their software tools can easily trace and recover data from even a formatted hard disk.
Hence both deletion and formatting are not a good idea in case you are selling off or donating your computer.
To completely remove the data, you need some data eraser tools. Tools such as CCleaner and Comodo System Cleaner offer you the facility of overwriting the data files that you are deleting. These programs, however, do not offer the facility of erasing the entire hard disk. They only erase the files related to your privacy
There are several programs available on the market for erasing the hard disk - both free and paid. For faster data wiping, you can call in security agencies that employ custom software. Of course, the cost will be higher when you use the services of a security agency.