Seagate on Monday announced that it has sold more than 1 million drives using a new recording technology that will offer consumers 5TB hard drives next year and possibly 20TB drives by 2020.

The technology Seagate is touting -- shingled magnetic recording (SMR) - is needed more than ever.

Just as NAND flash is running up against a miniaturization wall, where the circuitry has little room to continue to shrink in size, hard drives face a similar density dilemma. The data tracks on a 1TB hard-drive platter cannot afford to shrink much more, according to Seagate.

The theoretical limit of magnetic storage, called the superparamagnetic limit -- about 1Tbit per square inch of storage density -- is fast approaching. Increasing the density beyond that will lead to data corruption issues. Currently, Seagate's drives store data at up to 625Gbits per square inch of storage areal density.

SMR is only one of several technology advancements pushing the limits of hard drive capacity. Heat-assisted magnetic recording (HAMR) is expected to take disk drives to 5Tbits per square inch. Seagate rival Western Digital is expected to release helium filled disk drives later this year. The helium provides less resistance than air and so will allow more platters to be stacked closer together.

"With SMR technology, Seagate is on track to improve areal density by up to 25% or 1.25TB per disk, delivering hard drives with the lowest cost per gigabyte and reaching capacities of 5TB and beyond," Mark Re, Seagate's chief technology officer, said in a statement.

Seagate would not disclose which of its drive models today use SMR. It would only say that system makers that use them know they're using them.

How tracks are typically spaced using perpendicular magnetic recording.

The principle behind SMR is simple. With the technology, the tracks of a drive basically overlap like the shingles on a roof, thereby allowing Seagate to squeeze more tracks together.

The density problem came about when Seagate and other drive manufacturers pressed the limits of how close they could squeeze tracks together on a drive platter. For a mental picture of platter tracks, think of an LP vinyl record, except on a microscopic level.

The closer the tracks of a drive platter are squeezed together, the more data can fit in a disk drive. But, the closer together the tracks get squeezed, the greater risk of data corruption and read errors -- that is, the read/write head of a hard disk drive cannot discern the difference between tracks. In between the tracks are buffer areas to help the read/write heads track accurately.

Like stacking pancakes, more platters can be added to a drive, but that adds to the height of the drive.

Hard drive density

Hard drives today typically use perpendicular magnetic recording (PMR), a method of laying down data on a platter that creates tracks that are about 75 nanometers wide, which is smaller than a flu virus. Seagate introduced its first PMR drive in 2007. The drive, the Barracuda, held 250GB of data per platter. That grew to 1TB per platter by this year.