For some of us it is exciting when a new computer arrives but quickly we realise that we will need to install all our applications again and then copy all our documents to the new system.  This can be a huge task but once this is done the job is still not complete, we still need to erase the data from the old computer.

With identity fraud on the increase it is vital that you remove all personal data from a computer before disposing of it.

Many users think that deleting the files we don't want others to see is sufficient, this is not the case.  In most cases when you delete a file from a file system all that happens is the pointer to the data is removed, but the data is actually still intact on the disk.  There are many open source and freeware utilities that can 'un-erase' files which can be used to easily restore data you have erased.  There are others that think that a low-level format is enough to permanently remove all data from a disk, but a low level format only performs a single pass over the disk.  A disk that has been formatted can be analysed in a lab and often all the data can be recovered. 

There are however a number of options to securely remove data from a disk.  You could remove the disk from the computer and physically destroy it.  This is an extreme solution but is 100% foolproof.  Alternatively you could use shred.  Shred is part of GNU Core-Utils, and is therefore available on most live Linux distributions such as Trinity Rescue Kit (http://trinityhome.org/Home/index.php?wpid=1&front_id=12) or System Rescue CD (http://www.sysresccd.org/Main_Page).  Shred is a non-destructive process, it destroys the data but the disk is unharmed.  Shred runs several full passes using different data patterns over the specified file or device.  This makes it so the original data is unrecoverable as the physical sectors of the disk have been overwritten with different data so many times that there are no magnetic "echoes" of the original data left.  Shred works better with devices rather than files so my recommendation would be to 'shred' the whole disk.

You can boot the live CD and run fdisk -l to discover the hard disks found by Linux.  The addresses for a IDE hard disk will typically be /dev/hda, /dev/hdb, /dev/hdc or /dev/hdd depending on which IDE bus it is on and weather it is the master or slave.   A SCSI or SAS disk will be /dev/sda for the first disk discovered /dev/sdb for the second and so on.

The below example will 'shred' the device /dev/sda.

# shred –v /dev/sda

The v flag turns on verbose output so you can see the progress of the shredding.

WARNING.  Shred takes a long time to complete.  I recently 'shredded' a 120GB SATA disk in a 12 month old laptop, it took 25 hours to complete so be patient.