For most of the past 50 years, the areal density of hard disks—a measure of how many bits of data that engineers can squeeze into a given area—increased by an average of nearly 40 percent each year. Lately, though, that rate has slowed to around 10 percent. Everyone who works on magnetic storage is well aware of this problem, but only in the past year or so have executives from Seagate Technology and Western Digital, the leading manufacturers of hard drives, very publicly split on how to solve it. In back-to-back announcements in October 2017, Western Digital pledged to begin shipping drives based on what is known as microwave-assisted magnetic recording (MAMR) in 2019, and Seagate said it would have drives that incorporate heat-assisted magnetic recording (HAMR) on the market by 2020. If one company’s solution proves superior, it will reshape a US $24 billion industry and set the course for a decade of advances in magnetic storage. Companies that wish to store huge amounts of data do have other options, but hard drives are still the go-to choice for enterprise storage needs that fall somewhere between faster, more expensive solid-state drives built on flash memory, and slower, cheaper magnetic tape.