HDDs store repeatable runout (RRO) metadata (the share of the position error signal that is repeatable for every spindle revolution) as well as write operations metadata at the track level to account for increased adjacent track interference (ATI). Modern hard drives store gigabytes of metadata on spinning media since it cannot be cost-effectively stored in local DRAM and serial NAND. Western Digital's 20TB HDDs with OptiNAND technology are based on nine 2.2TB ePMR (energy-assisted perpendicular magnetic recording technology) platters, triple-stage actuator technology for more precise positioning of read/write heads, an iNAND UFS drive of unknown capacity that uses 3D TLC NAND memory, and the company's custom system-on-chip (SoC) that controls the drive as well as communication between the HDD and the EFD.
