The latest HGST - WD Ultrastar DC HC520 HDD | HUH721212ALE600 | 12TB 7.2K SATA 6Gb/s 256MB Cache 3.5-Inch Helium Data Center Internal Hard Disk Drive has “Power Disable” feature and won’t spin up on any legacy hard drive enclosure.
This is because the power to the pins is required to be different under the new specifications.
The “Power Disable” feature is a relatively new standard. Drives are supplied with an adapter disable the new feature,
……But then one doesn’t have a a convenient HD enclosure with simulated protection and heat dissipation.