In most operating systems, when a user deletes a file, it still exists on the hard drive but is made inaccessible through normal means. What’s actually deleted is the operating system’s pointer to the file, so it’s invisible to the user but still exists in storage. The storage space for the file is considered available and that may cause the data to be overwritten, although it is not possible to know that it has been.
It may be possible to recover the original data unless overwriting has taken place hundreds of times. Deleted files can be recovered through readily available freeware programs such as Recuva and Puran File Recovery as well as professional software such as EnCase that data recovery and law enforcement officials use.
File shredders are similar to hard drive overwriters except that they work with individual files, rather than effectively obliterating the entire drive. The programs overwrite the content in a file multiple times with zeroes, ones and random bits to ensure that the data is unrecoverable other than through extraordinary efforts.
If it’s crucial that data is obliterated, the only way to be sure it can’t be accessed is to physically destroy the device. The most effective way to ensure complete data destruction is to pulverize the assembly, in much the same way as a wood chipper disposes of tree limbs and brush. At a minimum, hard drive shredding involves severing every track on every platter in a drive. Hard drive shredding products and services exist for this purpose.