Western Digital this week officially introduced its first 20TB hard disk drives equipped with its OptiNAND technology that uses an iNAND UFS embedded flash drive (EFD) to boost performance, reliability, and usable capacity. Western Digital will offer 20TB HDDs in its Ultrastar family for hyperscale cloud datacenters as well as its WD Gold lineup for enterprises and even end users. The WD Gold 20TB is already available, priced at $679.99.Western Digital’s Ultrastar DC HC560 and WD Gold 20TB 3.5-inch hard drives share the same helium-filled HDD energy-assisted perpendicular magnetic recording (ePMR) technology powered platform with nine 2.2TB aluminum platters, a triple-stage actuator, and a 7200 RPM spindle speed. The ePMR platters feature a 1135 Gb/square inch areal density, which is a record for conventional magnetic recording disks (for now, of course). The drives are aimed at datacenters and enterprise-grade NAS, so they feature all the enhancements designed to protect them against vibrations in a bid to improve their reliability, longevity, and ensure predictable performance.In addition to unprecedented capacity, Western Digital's 20TB drives feature the company's OptiNAND technology that promises numerous additional advantages for capacity, reliability, and performance. In particular, OptiNAND offloads metadata from rotating media to the EFD (embedded flash drive), which frees up space on the disks and makes metadata related to repeatable runout (RRO) and adjacent track interference (ATI) available to the controller faster, which is good for real-world performance.Speaking of performance, Western Digital rates its 20TB drives for up to 269 MB/s sustainable transfer rate and says that their average latency is 4.16 ms. 269 MB/s is not the world's highest sustainable transfer rate for a 7200 RPM hard drive, but it will be interesting to see what actual advantages that OptiNAND will bring to these drives. As for power consumption, both WD Gold 20TB and Ultrastar DC HC560 consume 7W in operational mode and 6W in idle mode, which is just slightly higher when compared to Western Digital's 18TB HDDs that lack OptiNAND.While Western Digital’s Ultrastar DC HC560 and WD Gold 20TB hard drives share the same hardware platform, this does not mean they are completely the same. For one, they come with different firmware.Given positioning of these hard drives, Western Digital’s Ultrastar DC HC560 and WD Gold 20TB HDDs are rated for an up to 550 TB/year workload, come with a five-year warranty, and have an annualized failure rate of 0.35%. At least for now, the drives will only be available with a SATA 6 Gbps interface, but Western Digital may eventually offer the Ultrastar DC HC560 via it's HGST brand with a SAS interface as well.The WD Gold 20TB HDD is readily available directly from the company for $679.99. This is considerably more expensive than the company's WD Gold 18TB, which retails for $571.99, but it does add flash memory and should offer better performance. Those who buy top-of-the-range capacity points usually care more about capacity per rack and total cost of ownership throughout the lifespan of their drives rather than on immediate price.