To delete data from a hard drive properly, a rewriting process of all sectors is performed recursively. That is, the hard disk is completely rewritten repeatedly in a write loop, which makes the recovery of residual data to be an almost impossible task.
To delete data from a mechanical hard drive, the most appropriate tool, in our opinion, is DBAN (Darik’s Boot And Nuke). You can boot it from a bootable CD, or upload it to a pendrive with Unebootin or Yumi. You can start up from it and just have to wait to be able to choose the drive or drives that you want to safely erase. If you don’t want to complicate things, the standard method without any configuration already provides high quality erasing levels.
Simply select the drives that you want to delete and run the action. The standard method used is DOD Short. This method, designed years ago by the Department of Defense of the United States, uses a triple writing in each sector of the disk. First a character, its complementary character and then a definite a random character. Subsequently a check is made. It has been used for years and though some experts describe it as obsolete, it certainly is a fairly reliable method for the type of information that a home user would leave on its hard drive.
The process is slow, long, but the results will be those you seek. Data recovery after going through the whole process will be virtually impossible and only accessible with very advanced methods. This method does not damage the hard drive, so you can reuse it without problems. In fact, its reuse with new data with further enhance the removal of the previous.
One last tip. The overwriting or low-level format has been deprecated since almost 20 years ago. It’s not really useful for this and it does not serve to recover defective sectors on a modern drive.
SSDs behave differently and require specialized methods. On one hand, it is simpler, but it also requires a specialized tool since it is the manufacturer, in the disk design, the one who knows exactly the methods used for recording data in memory cells.
Whereas ATA, the standard agreed between the main actors of digital storage, has standardized the process calling it “ATA Secure Erase”. We usually know it only as “Secure Erase”. Each manufacturer implements it in a personalized manner following standard keys.
A good example of a specialized tool manufacturer is the Secure Erase offered by OCZ’s SSD Guru. You can use it from Windows or from a DOS start up. This tool will discharge all electrons from the hard drive cells, thereby eliminating any data stored in them. It takes just seconds to do so and its effects are permanent. My advice is to seek the manufacturer tool of your drive, as all or nearly all have this kind of utility tool and is often free.