The drive which you are going to sell or dispose of may still contain either some personal data. This data is to be removed off the disk before getting rid of it.
As far as software is concerned, there are several programs both free and paid that can overwrite the information by either 0s or random noise. Once overwritten, the data is irreversibly deleted.
To obtain the same result yourself as with the secure erase software, just format the hard disk and load it fully with some stuff you don't care about (similar to multiple copies of House MD videos). Once the big file fits no more, continue adding smaller files. You can also do this with an USB box. On modern full encryption disks, just changing a password gets you the same result as after a secure erase.
