How much monetary value can a second-hand 120GB USB drive have nowadays? It's not the drive, it's the huge superconductor magnets you need to destroy the micromagnetic traces of bits written to the drive in the past.
There are much cheaper ways to do that (a blowtorch or a bucket filled with strong acid to dissolve the plates), but that was not my point. Before spending any amount of time trying to erase all traces of data that were once on a hard disk, you should consider if it is worth the effort. After overwriting the data once with random data, the drive itself (without tampering) won't reveal any of it (otherwise, it would not be able to hold any data after being written once). If the data that was once on the disk requires you to spend more effort on it (to ward off more sophisticated methods of data recovery), scrap the thing and buy a new one. Even at the hourly wage of someone flipping burgers, you will quickly spend more than a second-hand 120GB USB disk is worth.