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The Pesky PWDIS Feature In Newer SATA Specs

A legacy SATA connector with P3 assigned to the 3.3V rail

During our (ongoing) evaluation of a Seasonic Prime Ultra power supply (PSU), we noticed a small detail that previously escaped our attention: the existence of the power disable (PWDIS) feature in HDDs that follow the newer SATA 3.3 (announced on February 16, 2016) and SATA 3.2+ specifications.

This feature utilizes the third pin (P3) of the SATA connector, which in the older SATA specifications is tied to the first (P1) and second (P2) pins, and all of them carrying the 3.3V rail to the drive in use. According to the newer SATA specs, P3 is now independent and transfers the power disable signal. This means that if you combine a PSU with the older SATA connectors, featuring 3.3V at P3, and a new HDD that supports the power disable feature, the HDD won't ever start because it will see a continuous high-state signal on P3. It will get stuck in a hard reset condition, which will prevent the HDD from spinning up.

The combination of legacy SATA connectors with new HDDs supporting the PWDIS feature can cause major headaches to users, but the fix is easy, at least if you don't have many of those new HDDs to power. You can totally bypass the 3.3V rail (which after all isn't used in HDDs) by using a simple 4-pin Molex-to-SATA adapter to supply power to the HDD. This adapter will remove the 3.3V signal from P3 of the SATA connector, and the HDD will start normally; however, there will be no PWDIS support.

Moreover, all modern PSUs have a limited number of 4-pin Molex connectors, compared to the SATA models, so if you need to power a large number of HDDs, you'll probably get into trouble. Finally, in general we advise against using any cable adapters or extenders in PSUs, because low quality adapters/extenders featuring thin gauges can melt and cause dangerous short circuits.

A SATA Connector That Supports Power Disable (PWDIS)

Speaking of the PWDIS feature in the 3.3 and 3.2+ SATA specifications, its purpose is to allow users to manage the power consumption of SATA devices remotely and also provides the ability to hard-reset the drive from a distance in case you need to power cycle it. This feature is mainly designed with business environments in mind, where the HDDs are installed in storage enclosures located in remote facilities.

Currently, there are a number of HDDs supporting the power disable feature, and that number will only continue to grow. Some companies also offer the corresponding products in two versions, one with PWDIS support and one without, in order to avoid compatibility problems. Further, if you're looking to buy HDDs for NAS servers, you should also pay attention to this issue and check to be sure your hardware is PWDIS compatible.

You will find more information about the PWDIS feature in this tech brief.

To sum up, for products supporting the optional SATA 3.3 power disable (PWDIS) function, the third pin (P3) of the SATA connector is now assigned as the Power Disable Control pin. If P3 is driven HIGH (2.1V-3.6V), the power to the drive circuitry will be cut. All drives with this optional feature will not power up if a legacy SATA connector is used. This is because P3 driven HIGH will prevent the drive from powering up. The easy, and not so elegant, solution is to use a 4-pin Molex to SATA connector or a power supply equipped with SATA connectors that follow the SATA 3.3 specification.

Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.