It is not sufficient to simply delete via the recycle bin or by formatting
With computers, files are generally moved to the recycle bin first. From there, you can recover your data if needs be, and they are seemingly deleted for good once you empty the bin. The latter however does not really “delete” the actual files, but only the directory link to the file. This renders the file “invisible” to users, and those areas of the hard drive containing the files to be deleted are earmarked for overwriting. These data keep existing until another file is written to the area marked for overwriting.
It is a similar case when formatting data carriers. When quick formatting, references to all files are removed from the directory. Yet, the file contents survive with this process - even if in the shape of orphaned files.
It is more effective to run a complete disk format. With today’s operating systems, this will completely overwrite all storage spaces with zeros. It is therefore de facto impossible to recover any files by reasonable means.
You can therefore recover deleted data which have not been overwritten. This can be very helpful if you inadvertently delete a file you still need. However, for security reasons - for instance if you wish to delete a confidential file for good - this is not desirable.
To delete an individual file or a complete data carrier for good, you may need special software. The process depends on the type of the data carrier or the type of recording process used here:
Magnetic hard drives
On magnetic hard drives, the filing location of any file is precisely defined. Special software is therefore able to locate this specific hard disk area and to overwrite it - usually even several times, to be on the safe side. This process will erase data for good.
If you are thinking of disposing of or selling your computer, you should either remove its data carriers or at least make sure you delete all data on its hard drive. After all, you really don’t want the buyer of your device to be able to retrieve your sensitive data. It is easiest to use a bootable CD with suitable tools which will overwrite the whole hard drive, for instance DBAN for Windows.
USB sticks and SD cards
For technical reasons, on so-called flash storage media such as USB sticks or SD memory cards, it is possible for the same contents to be stored in several filing locations. This results in the automatic creation of copies. When deleting by overwriting, only the copy last used will be deleted - the others remain.
You should therefore note that you can only ever securely delete data from a flash storage unit by irrevocably erasing the whole medium. There is basically no way you can securely delete individual files from USB sticks and SD cards.
SSD hard drives
Files on SSD hard drives now built into newer computers can therefore not reliably be deleted with the software mentioned above. This has technical reasons: To ensure the memory cells wear out evenly, the stored contents on this hard drive are automatically reorganised from time to time. This creates “lost” data copies which cannot be specifically overwritten. It is therefore not possible to reliably delete data by overwriting.
Some SSD hard drive manufacturers offer integrated functions which find and purportedly irrevocably delete such lost data on this type of data carrier. However, it is nigh on impossible to check that is function actually works and is really reliable.
In addition to the physical destruction of the data carrier, it is the same here with regard to files. You can only securely delete them if you delete the whole storage area of this data carrier.
Another secure alternative is to encrypt individual sensitive files or even the whole storage area of this data carrier to start with. Without key material, third parties are then unable to read confidential contents. This also has the advantage that your sensitive data are even protected if your device (for instance your laptop) is stolen or lost - no access without your key!
Optical storage media
With writable optical storage media, a laser engraves data into a reflective layer in a hole pattern. Depending on this layer, you can either repeat this process just the once (R) or several times (RW).
Due to the technical difficulties and the low value of these data carriers, it is the most practical solution to destroy these data carriers to delete your data.
Magnetic data tapes
Magnetic data tapes are often used to back up a whole data collection to retain them over extended periods of time. They therefore enable “looking into the past” - to even retrieve data long believed lost.
Magnetic data tapes back up contents to be stored in sequential data sets. These generally form an unchangeable unit provided with integrity protection. You cannot delete individual files from these. When deleting data, you have to destroy the whole data set instead.