Securely erase hard drive in Linux

# shred -f -z –iterations=2 –random-source=/dev/zero /dev/sdc
-z will zeroize the hard disk at the end. You can also use –random-source=/dev/urandom, to use random numbers. Only it will take longer.

Alternatively, you could use the following command:
# dd if=/dev/zero of=/dev/sdc OR
# dd if=/dev/urandom of=/dev/sdc