Re: it's part of the the SATA specification.
3.3V in the SATA spec, I should make clear. The specification for the SAS connection had them as N/C, so they would float anyway. The whole thing just doesn't make any sense to me. And it is claimed that the SCSI Trade Association determined that writing the spec the way it did wouldn't be a problem.
"With the introduction of 12G SAS, a new SAS standard, SAS-3, redefined P3 (Pin 3) from “3.3V Power” to “POWER DISABLE”, i.e. “Reset”. At that time, the STA (SCSI Trade Association) researched the marketplace and determined that there were no conflicting legacy concerns."
WD were key contributors to that association and the standards forum. Their suggested solution for those that were silly enough not to spot that HUH721010ALE604-0F27606 was so radically different to HUH721010ALN604-0F27609 when the textual description of the two variants in the reseller channel is actually identical, is that you use a MOLEX adaptor. Which isn't really an option if you have a backplane connector. Worse, some hot-swap backplanes actually work fine with these drives until you power cycle the device and it puts the expected 3.3V onto that line during the POST and enters an error state as the expected drives don't appear or it bums out as there's a shadow BIOS (or equivalent) on the drive itself which is supposed to load and fix that particular quirk.
One really wonders about the thinking behind this... I mean, just why? What was the reason? If there is a good reason, it'd be more acceptable, or at least understandable. Why isn't there a jumper for PWDIS or (overline) PWDIS? Or a DIP switch? Or even a register option somehow?