Esta é uma pergunta semelhante a Como limpar espaço livre em disco no Linux?
Aqui está a resposta que eu dei então;
You can use a suite of tools called secure-delete.
sudo apt-get install secure-delete
Isso tem quatro ferramentas:
srm - exclui com segurança um arquivo existente
smem - excluir com segurança os rastros de um arquivo de ram
sfill - limpe todo o espaço marcado como vazio no disco rígido
sswap - limpe todos os dados do seu espaço de troca.
Na página man do srm
srm is designed to delete data on mediums in a secure manner which can not be recovered by thiefs, law enforce‐ment or other threats. The wipe algorythm is based on the paper "Secure Deletion of Data from Magnetic and Solid-State Memory" presented at the 6th Usenix Security Symposium by Peter Gutmann, one of the leading civilian cryptographers.
The secure data deletion process of srm goes like this:
- 1 pass with 0xff
- 5 random passes. /dev/urandom is used for a secure RNG if available.
- 27 passes with special values defined by Peter Gutmann.
- 5 random passes. /dev/urandom is used for a secure RNG if available.
- Rename the file to a random value
- Truncate the file
As an additional measure of security, the file is opened in O_SYNC mode and after each pass an fsync() call is done. srm writes 32k blocks for the purpose of speed, filling buffers of disk caches to force them to flush and overwriting old data which belonged to the file.
Espero que isso ajude.