Use the shred utility to wipe an old hard drive. Shred is part of the coreutils package on Linux.

# dpkg -S /usr/bin/shred
coreutils: /usr/bin/shred

To wipe /dev/sdc use this:

shred -vfz -n 20 /dev/sdc

This will overwrite the drive 20 times (-n 20), show progress (-v), force if permissions needed (-f), and overwrite the last time with zeros (-z) making it look like a blank hard drive. 20 times is way overkill, but we have to be paranoid. The average person wouldn’t be able to get past the layers of RAID1, volume encryption, lvm, and ecryptfs home dir encryption, let alone a single pass of zeros, but we want to make it more difficult for people/agencies who know what they are doing.