What we think of as the material “real” of the computer – that is, the binary itself, the ones and zeros – is really a fiction. Depending on the media involved, the one or the zero is implemented as a level of relative difference in amplitude (in volatile memory) or magnetic charge (in storage). The material truth of a bit is still analog, continuous information (typically the difference between a charge of about .4 of charge capacity for a memory address and a charge of about .7 or so.)
In hard drives, the act of writing and rewriting leaves a bit of difference, a variance that can be decoded. In other words, if you write something to your hard drive that turns a bit to zero, and then turn it to 1, and then back to zero, there is actually a difference (in terms of the charge of the magnetic medium) between the first instance of “zero” and the second instance of “zero.”
It is a demanding and expensive process, but in theory, the slight differentials in the charges of the elements of the magnetic medium can be deciphered, and data that was over-written several times over recovered. There are practical limitations to this; however, while the fiction of the binary, of the pure 1 and the 0, is capable of instant oblivion, the gap between the material and the metaphor hides a surprising persistence of lost memory.