There are many app's on the market, some free, some commercial, that will erase data on the hdd using the pattern change algo that will remove any residual magnetism and effectively stop anyone from reading the content. Problem is, takes hours to delete a big HDD. The simple few hundred meg file takes dozenth of minutes.
The "quick eraser" is the effective and speedy way of doing the same thing. The problem is that it will remove WHOLE hdd, you cannot selectively delete "part" of the hdd, nor you can select hdd if few of them are near each other. It works by creating strong magnetic field. You can purchase "quick eraser", 110/220V AC powered or 12/24V powered in huge number of stores (especially in stores specialized for storage media's as those erasers were used quite a lot with old tape medium).
You can also create it your self. Even better, I saw "handmade" solution once as a part of security system. Make a cardboard shell around your HDD, then wind wire around it, few tens of thousand winds. Connect that wire to main power (AC 110/220V) trough relay. Control relay from external control box. When main power start flowing trough windings around the HDD takes few seconds before HDD is irrecoverably deleted. Now, there is many things that can go wrong here, too few windings and you will just short the main power so fuze will blow and you might even start a fire.. the magnetic field is quite strong so it might affect other media in the room.
Now, I just saw this from distance, there might be more to this, maybe there is a bridge rectifier on the input so there is actually 380V DC going trough the windings, some resistor might be there to limit the current ... what I remember is that guy who made it told me that he used the same principle CRT degauss works, so one can google for CRT degauss and find the actual schematic
Anyhow, the eraser works
tried/tested few times, not even the magnetic microscope is able to read data from the HDD after that. Check out the specialized "storage" stores, you can get specialized HDD cases that protect HDD from external magnetic interference, you can get cases with built in eraser, you can get almost anything you can think of, crazy world out there so many "strange" people around wanting strange things hence there is a whole techno coverage of that segment.