When data is deleted from a hard disk the actual 1's and 0's are not
removed from the disk, as such. Instead the directory information
about the name and location of the file is lost and the area where the
file was stored is made available for overwriting by other data. This
means that the original data can be recovered even though the file has
been deleted (but possibly not yet overwritten).
This is great if you have accidentally deleted data and need to
recover it. There exist forensic tools to do this. A technique called
Magnetic Force Microscopy (MFM) allows, at a cost, the recovery of
even the last two or three layers of data as written to disk! But
today's hard disks are also clever in dealing with bad blocks and may
unwittingly store some of your data permanently in places that no one
generally has access to!
However, if you are wiping a disk for return to someone else, or never
plan to use the disk again, you may want to ensure that you have
completely removed any trace of your possibly confidential data from
the disk. The wipe
package in Debian GNU/Linux provides a
tool to do this.
$ wipe -kD /dev/sdb1
$ wipe -kD /dev/sdb2
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