Before throwing away your drives, always ensure that personal data (including students works, list of seminar attendance, student grading — see the Data protection officer guides) are not stored there.
Ideally, always completly erase the drives. However, common erasing and/or formatting the drive is usually not sufficient).
For securely erasing drives, the freely available program Darik's Boot And Nuke (DBAN) can be used.
To use it, the steps are as follows:
