Just when you think SSDs are poised to catch up to HDD capacity (albeit at a price premium), news comes along that mechanical drives still have some life in them yet. Seagate has produced the first 1Tb per square inch platter using Heat-Assisted Megnetic Recording (HAMR) technology, a 55% increase over current drives on the market.
Drives sold today use a process called Perpendicular Magnetic Recording (PMR), which made its debut in consumer gear in 2005. This tech stores data perpendicular to the normal of the platters surface rather than in a longitudinal pattern, allowing more bits to be represented over the same surface area.
HAMR differs from PMR in that it uses localised heat to allow for higher coercivity materials to be used in platters, without requiring a strong magnetic field to write bits. High coercivity materials allow smaller bits to remain stored reliably, because they resist demagnetisation. Essentially, coercivity is a measure of resistance to becoming demagnetised - something you certainly don't want happen on a hard disc. The location being written to is heated above the material's Curie temperature which decreases coercivity, thus allowing it to be written using a smaller magnetic field. Smaller, more reliable bits equates to higher density drives.
This new technology is expected to double the capacity of 3.5" hard discs from 3TB to 6TB. Similarly, 2.5" drives will rise from 750GB to 1.5TB per drive.
HAMR technology has a theoretical limit in the range of 5 to 10 terabits per square inch, so we can expect to see further growth as production processes improve, and more research is performed.