Researchers discovered that magnetic avalanches may pose a serious threat to data stored on hard drives as the storage densities continue to increase:
Hard drives store data by magnetizing small clusters—called domains—of atoms that sit on rapidly-spinning platters. The magnetic effect is created when an external magnetic field aligns the rotation and spin of electrons in the atom. While the spin of an electron is not physically the same as that of a rotating body such as a planet or top, scientists have found that it can exhibit similar characteristics. One of these is precession, the wobbling effect seen in spinning tops as they lose energy, and the cause of a 25,765-year cycle on Earth that slowly changes the direction of the north and south axis.
Electrons that have their spin direction modified exhibit precession effects for a few nanoseconds, before they settle down and continue their quantum mechanical lifestyles. However, during this period of precession, the electrons can exert forces on neighboring atoms that could cause those atoms' electrons to flip spins as well, triggering an "avalanche" of bit-flipping that only dies down due to the damping effect of the physical material on the platter.
As PC World noted, Deutsch and Berger's letter suggests that today's hard drives are mostly immune to runaway avalanches because of this damping effect, something achieved by trial and error over the years as manufacturers found out what materials made for reliable hard drives. However, as drive storage densities continue to increase, the problem could start to reassert itself unless more research is done to find the best materials for damping magnetic avalanches.