The problem is that the HDD is designed, given the head, recording signal, and surface material, to only support the original capacity under the signal theory that covers the current method of recording. It does NOT matter that in theory, the disk material MAY be able to save far more data with a different head, and signal method. Only the current method matters. But the owners of Slashdot will allow periodic FUD articles to appear that DISCOURAGE people from using proper file erase tools, on the basis that its actually a waste of time, because the NSA can still get your data no matter how you erase it.
You sure YOU don't work for the NSA? The recording capability is what it is, but the reading capability is whatever you can put in a $100 consumer drive operating at 100MB/s with 1 error in 10^14 bits accuracy. What you can do with a >$1 million electron microscope at 1/1000th the speed at 1/1000th the accuracy is another matter. You might not want a 0.1 MB/s drive that corrupts a bit every megabyte but for forensics that's plenty. Never mind that all modern drives just pretend to offer you a linear disc, in reality it remaps a whole sector if a single bit fails. How much compromising info can you write in 4023 out of 4024 bits of a 4K sector? It's not useless but everything you hope to achieve with erasing is better achieved with encryption. Nor are they mutually exclusive, if you want to wipe your encrypted drive for that extra unrecoverable feeling go ahead.