An interesting question came up on StackOverflow: does a hard drive weigh more when it’s full than when it’s empty? Or more generally, does the weight of a hard drive change depending on how much (and what) data is stored in it?

First of all, as far as anyone in the IT industry is concerned, the answer is no. Any change in mass that would result from magnetic alignment is far too small to be measured by even the most sensitive scales in the world — we’re talking about a difference of something like \(10^{-14}\) grams.

Now, how did I get that number?

Let’s start from the beginning. Every atom has a property called the magnetic dipole moment, which means it acts like a tiny bar magnet, with a north pole and a south pole. In a ferromagnetic material, the type that’s used to store data in a magnetic hard drive, adjacent atoms tend to align parallel to each other, so that their north poles all point in the same direction. This leads to the formation of magnetic domains, small groups of atoms which are all aligned; each domain acts like one tiny bar magnet …