Seagate currently predicts it will be able to offer 36TB HDDs in 2021/2022 and 48TB in 2023/2024. HAMR is anticipated to enable areal density to double every 2.5 years, which may result in 100TB HDDs beyond 2025.
To improve performance, Seagate is developing multi-actuator technology. The first drives with dual-actuator technology are expected in 2019, these will be PMR-based disks with a capacity of 14TB. HAMR will feature product lines with single and multi-actuator, the first will be aimed at capacity, while the latter will be aimed at performance. Full details at The Register.
Looking into the future, Seagate still sees a role for HDDs. Once HAMR is no longer able to provide improvements in areal density, the company can switch to HDMR (heated dot magnetic recording):
When HAMR tech loses its ability to advance areal density further, Seagate envisages using HDMR (heated dot magnetic recording), a refinement of HAMR that uses BPMR (bit-patterned media recording) in which smaller bits (made up of individual magnetic grains) sit on raised dots or islands a few nanometres above a baseline. That would effectively increase the separation between neighbouring bits.