Seagate Demonstrates HDD with PCIe NVMe Interface [tomshardware.com]
Seagate has demonstrated [seagate.com] the industry's first hard disk drive connected to a host using a PCIe interface at the Open Compute Project Summit. Like solid-state drives, the experimental hard drive uses the NVMe protocol to operate alongside SSDs seamlessly. Usage of a single protocol for different types of storage devices will greatly simplify datacenters.
The experimental SSD is based on Seagate's proprietary controller that supports all three major protocols, including SAS, SATA, and NVMe over a 'native NVMe port,' and does not require any bridges.
[...] Modern HDDs can barely saturate even a single PCIe 2.0 link, but future multi-actuator HDDs promise to be much faster, so 6 Gbps provided by SATA or 12 Gbps offered by SAS might not be enough at some point. To that end, the industry has to think about future interfaces to connect hard drives, and PCIe seems like a natural choice. Furthermore, as SSDs are gaining traction in datacenters, the NVMe protocol becomes pervasive, so it makes sense to adopt it for HDDs. This is why NVMe 2.0 [tomshardware.com] adds hard drive support.
NVM Express aka Non-Volatile Memory Host Controller Interface Specification [wikipedia.org].
Also at Phoronix [phoronix.com].