Secure erase procedure (also known as a "disk wiping") is
performed when it is for some reason necessary to irreversibly delete
some data. This procedure is typically applied to "sensitive" (i.e. top secret)
data only. When you erase the file by using normal means, the corresponding
references are deleted and the space formerly occupied by the file is marked as
being free. However, the actual file data remains intact on the disk until the
space is claimed by some other file (at which point the original data is
overwritten). We consider data overwritten at least once beyond any recovery.
While it was possible to recover previous layers of data from older
generation media (like magnetic tapes), we consider this is impossible with the
modern hard drives. Rumor has it the other way, referring to some obscure
"government agencies" being able to recover previous layers of data. We believe
such references are a hoax. For any kind of electronic memory (i.e. not
involving magnetic surfaces, examples being RAM and SSD devices) the recovery of
overwritten data is certainly impossible, because no remains of previous state
are available by the very design of the storage.
There is a DoD (U.S. Department of Defense) standard of data wiping - DoD 5220.22-M -
described at www.dss.mil.
When working with a disk wipe tool, always keep in mind that the deletion is
really irreversible. There is no way back if you delete wrong files.
When a need arises to perform a secure erase, we use SDelete from SysInternals.