There are many security and privacy reasons you might need to do a full disk erasure to completely wipe off your data. Here are some ways to nuke all traces of your data once you have decided:
- Darik’s Boot And Nuke (DBAN): It started out as a program on floppy but now it comes in ISO format and you can burn it in a CD. To install it on a USB drive, check this thread.
- Securely wipe disk (article from Arch Linux forum)
The idea is simple enough though. If you are nerd enough you can do this without any additional software installation by dd-ing out everything with random data from /dev/random or /dev/urandom on Linux. Overwriting only once is enough. Make sure you erase the MBR (first 512 bytes on disk) or GPT structures (first 1 MB on disk).