Trying to provide some sanity:
'Formatting' the drive (wiping out the file tables, usually) does, essentially, nothing. Data recovery software can generally pull your files back up unharmed, unless the drive has already been completely refilled with new data.
Doing a zero-fill (or writing random data over the entire drive sector-by-sector) will prevent data recovery software from working. It will just read back all zeroes, or random junk. HOWEVER, drives processed like this can still often be salvaged by taking apart the drive in a cleanroom and scanning the platters with a much more sensitive magnetic scanner than is present in the drive itself. This is because data that was stored statically on the drive for a long time slightly changes the 'baseline' magnetic field strength of the media.
Writing random data over the drive multiple times (as recommended by the DoD) will make this sort of process much more difficult, since any traces of old magnetic fields will largely be obliterated when the sectors are repeatedly overwritten. HOWEVER, there are other methods that can still be used -- for instance, a drive's calibration may change slightly over time (or with changing temperature). This means data that was written a year ago may not be completely erased when you zero-fill the drive today. Again, sensitive scanners can be used to try to pick up the unerased edges of the tracks.
EXTREMELY powerful degaussing (nothing you have access to in your home will do, though very strong magnets might if you took the drive apart) will destroy the low-level formatting on the platters. This *will* render them essentially unreadable, since there will no longer be any indication of where the invididual data tracks start and end. However, it may still be possible to recover some data if the degaussing was not complete.
External physical damage to the drive will often barely damage the platters. Unless you physically disassemble the drive and destroy the platter surface (such as by grinding them down), often the drive can be carefully disassembled and at least some of the data recovered.
Complete physical destruction of the drive (usually by melting it down, or grinding down the platters) guarantees nothing will ever read that data again.
That said, a zero-fill or writing the whole drive with random data (once) will keep any 'casual' snoopers from sifting through the contents of a used hard drive. Accessing any data once that has been done requires professional expertise and expensive equipment. Unless you have corporate trade secrets or some other extremely valuable/sensitive data on your drive (or you have reason to believe someone might specifically target you in an attempt to get your data), more extreme measures are a little silly.
