To delete data on a hard disk safely, a normal formatting is usually not sufficient. Data fragments can often be recovered afterwards.
Programs like Darik’s Boot and Nuke (DBAN for short), which is also free of charge in the normal version, are better suited for this. Here you can set how often a disk should be overwritten (the more passes, the longer the process logically).
The program can be downloaded ISO file, which is then installed on a bootable medium, such as USB stick. This is very easy with the free program RUFUS possible. This not only creates a bootable medium, but also mounts the corresponding ISO file.