Here a quick note on how you can completely wipe your hard disk in Linux.
Without mounting your harddrive overwrite all content on it with random data. Make sure to choose the right harddisk (im my case sda but it could also be sdb, sdc, …)! Please note that erasing a a 1000 GB disk takes about 24 hours with this command.
time dd bs=1024k if=/dev/urandom of=/dev/sda
- Overwrite the content again, this time with zeros. This is way faster than using urandom but will still take around 8 hours for a 1 terabyte hard drive.
time dd bs=1024k if=/dev/zero of=/dev/sda
For most users it is probably enough to omit step 2 and only overwrite your content with zeros as in step 3, see the discussion here. Another nice article about dd and how it can be used to increase the effectiveness of encryption can be found here.