32

I’ve found diverging instructions on the ’net about this.

To recap: SATA with the 4-pin Molex (white) power plug is not hot-pluggable, but either the wide connector or the separate (15-pin power and 7-pin data) connectors are.

However, in which order do I plug the cables in? Asrock says to connect first the data cable to the mainboard, then to connect, in this order, the power cable, then the data cable, to the drive. Another hardware guide says to connect the data cable first.

It’s surprising that a definitive answer on this is so hard to find.

  • You don't plug the cables manually at all. Hotplug is only supported when using a suitable backplane/drive chassis with a combined connector. – JamesRyan May 8 '15 at 15:57
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    @JamesRyan that’s wrong, for SATA. – mirabilos May 8 '15 at 16:00
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    you are wrong, please see the SATA interoperability spec. For hotplug to work correctly it must be ensured that they are not inserted at any angle for the proper contact order of the staggered pins. – JamesRyan May 8 '15 at 16:09
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    @HBrujin this is about a system in a business environment. – mirabilos May 10 '15 at 12:17
  • Back when I was in a government environment I remember that the only reason we needed the special hotplug carrier was to make sure that the power fingers engaged before the data. They were just about an eighth of an inch longer to ensure it was in that order. I wonder if t might be the same for SATA? Also, I wonder if it is OS-dependent. FYI, I found this question 2 years and 1 month after it was posted. Gotta love this database. – SDsolar Jun 24 '17 at 21:20
44

In the SATA specification this is referred to as hot plug and hot removal and they are two separate events. While the electrical and communication layers support both hot plug and hot removal, check that your drive controller, operating system, and drivers support them.

Note that all of the below ONLY applies to host and devices (ie, drive controllers and drives) that BOTH declare they are hot plug capable.

If your drive controller has specific instructions, follow them. If not, read on.

It doesn't matter which plug to attach first. SATA drives are allowed to be connected to data without power, and to power without data. They are designed so when data is connected without power, some limited drive information can still be obtained (this is mostly used in RAID and backup setups where you want to keep some disks offline to reduce wear and tear, but still need to know what's installed).

So if you plug in the power first, the drive turns on, recognizes there's no data cable, and waits for the data cable to be attached.

If you plug in the data first, the computer recognizes the drive attachment, and that the drive isn't ready, and waits for the drive to signal that it's available.

If you do happen to get a single cable with both power and data, though, you'll find that the data pins are further behind the rest. The pins are staged as follows:

  • Ground and precharge inrush power
  • Power
  • Data

This suggests that while the drives and controllers should support plugging either cable in any order, when they have control over how cables are connected they prefer power before data.

So if you wanted to be pedantic and prefer one order above the other, your best bet is to follow what they do and connect the data cable last. Note that disconnecting the data first, then the power, when removing the drive will allow the drive to detect the removal, and possibly perform a few last millisecond housekeeping tasks before the power is fully removed.

But, again, the specification allows connection in any order, and should work fine in any order.

Specification excerpts

From SATA revision 3.0 June 2, 2009 Gold Version

4.1.60 hot plug

The connection of a SATA device to a host system that is already powered. The SATA device is already powered or powered upon insertion/connection. See section 7.2.5.1 for details on hot plug scenarios.

You might think the above suggests that power should be applied first or simultaneously, but this is clarified in 7.2.5.1:

7.2.5.1 Hot Plug Overview

The purpose of this section is to provide the minimum set of normative requirements necessary for a Serial ATA Host or Device to be declared as “Hot-Plug Capable”. As there exists various Hot-Plug events, there are relevant electrical and operational limitations for each of those types of events. The events are defined below, and the Hot-Plug Capability is further classified into:

a) Surprise Hot-Plug capable
b) OS-Aware Hot-Plug capable

When a Host or Device is declared Hot-Plug Capable without any qualifier, this shall imply that the SATA interface is Surprise Hot-Plug Capable.

For the purposes of this specification, Hot-Plug operations are defined as insertion or removal operations, between SATA hosts and devices, when either side of the interface is powered.

...

Hot-Plug Capable Hosts/Devices shall not suffer any electrical damage, or permanent electrical degradation, and shall resume compliant Tx/Rx operations after the applicable OOB operations, following the Hot-Plug Events.

Here's the key part of the specification you're interested in. All the following situations shall not damage the device or host, and both the device and host shall resume normal TX/RX communication after any of the following events. While these discuss specific architectures (backplanes, for instance) the drive and host themselves are electrically and otherwise the same - these are merely methods of connection and there's no practical difference between them and your individual cable scenario:

Power remains connected while data is plugged/unplugged

  • Asynchronous Signal Hot Plug / Removal: A signal cable is plugged / unplugged at any time. Power to the Host/Device remains on since it is sourced through an alternate mechanism, which is not associated with the signal cable. This applies to External Single-Lane and Multilane Cabled applications.

Data is connected where power is not available

  • Unpowered OS-Aware Hot Plug / Removal: This is defined as the insertion / removal of a Device into / from a backplane connector (combined signal and power) that has power shutdown. Prior to removal, the Host is placed into a quiescent state (not defined here) and power is removed from the backplane connector to the Device. After insertion, the backplane is powered; both the Device and Host initialize and then operate normally. The mechanism for powering the backplane on/off and transitioning the Host into/out of the “quiescent” state is not defined here. During OS-Aware events, the Host is powered. This applies to “Short” and “Long” Backplane applications.

There are two other situations here which don't apply to this question. Read the spec for more.

However, they do provide the following warning in the specification:

NOTE: This does not imply transparent resumption of system-level operation since data may be lost, the device may have to be re-discovered and initialized, etc. Regardless of the above definitions, the removal of a device, which is still rotating, is not recommended and should be prevented by the system designer.

In other words, the hot removal capability is the responsibility of the system designer, and they should ensure the drive is stopped before hot removal occurs. You, in this case, are the system designer. If your OS and driver don't have a mechanism to allow you to turn off the drive before unplugging them, then you aren't providing adequate hot removal support, and should not perform hot removals on the system. This is tackled by manufacturers by providing locking or handled drive cages where the lock to remove them tells the OS to perform the drive shutdown, or pulling the handle out a short way does so. The user is then instructed to wait for notification that the drive can be removed (usually an LED on the drive carrier itself).

  • Excellent answer. This is exactly what I remember about SCSI. Sounds like with SATA it is best to have power before data. – SDsolar Jun 24 '17 at 21:22
  • I would donate reputation to this answer if it were possible. – Daniel F Oct 15 '18 at 12:37
5

It is theoretically best to connect the power cable to the hard drive to allow the hard drive the brief second to initialize, but this is seriously only a brief 'moment'. If you look at the power connectors on a SATA 15-pin power cable and on the 7-pin data connector you will notice that some of the traces extend out slightly, this allows the powered connections to make contact before the data connections do.

Here is an example: http://www.satacables.com/assets/images/sata-power-cable-extension-8-inch.jpg

So the long story short is it makes little difference on which physical cable you plug in first as the SATA connectors have thought this issue through for you. You could technically plug them both in at the same time and have perfectly normal results (as would be the case when plugging into a hot swap drive bay).

  • Yes, plugging them in at the same time is the optimum. But do I connect the data cable first (so that the drive can say hi to the host as soon as it has powered up and initialised itself), or the data cable last (so that the host doesn't detect a “problem on the bus” before the drive is powered up, possibly disabling the bus)? Assuming a human plugging them in will make a 2-3s difference… this is eternity to a computer. – mirabilos May 8 '15 at 14:13
  • It really makes no difference as the cables take care of the power vs bus issue for you. If I was going to do one first it would be the power. That way I can make the sure the drive itself spins up and is powered on before I plug it into the data. On an SSD it makes 0 difference. – Atari911 May 8 '15 at 14:27
  • Do you have an authoritative reference for this? I find it especially hard to believe the first sentence, as they are two completely separate cables… – mirabilos May 8 '15 at 14:43
  • I guess the other answer added one to their post: ask.adaptec.com/app/answers/detail/a_id/17175/~/… – Atari911 May 8 '15 at 15:02
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    @mirabilos, both the power and data cables have extended ground contacts that mate before the other contacts. This is to allow the drive's potential to match the power supply's ground reference before the other voltage lines and the data lines are connected, and to mitigate voltage sags on connection (the 12V line is connected second, through current limiting resistors). – sleblanc May 8 '15 at 16:10
4

This question would get more answer and attention on superuser's section.

For your question, I would follow the manual from your motherboard (asrock). Usually the harddrive will warn up when you plug it, thus it's more logic to plug the Ac before, and after the data cable, but honestly I don't think there is a order, as hdd caddy do plug them at the same time when you slide them.

The SATA specification offers inherent support for hot-swap by using staggered pins for the device which ensures that the power signals are mated in the correct sequence to allow the drive to power up. The staggered pins implementation runs in the following 3-stage connection: First, the ground pins connect in order to allow for the electrostatic discharge, secondly the pre-charge pins connect in order to limit inrush current during device insertion. Finally, the power and data pins connect. (http://ask.adaptec.com/app/answers/detail/a_id/17175/~/principles-for-using-hot-swap-with-sas%2Fsata-systems)

  • Huh, I thought superuser was for root user questions (more software) and serverfault for server questions (more hardware)? Note that Asrock is not the manufacturer of the mainboard in question (it doesn’t come with any documentation), just one of the few sources of this information in the ’net. – Same time isn’t possible when there are two cables. I would have connected the data cable first personally. The people whom I asked are equally divided, so I seek for a somewhat-authoritative answer. – mirabilos May 8 '15 at 13:45
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    @mirabilos, Server Fault is for professional sysadmin questions, Super User is for general computer questions, roughly, see the help section on each site. Both have hardware and software questions. – dsolimano May 8 '15 at 13:51
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    Well, this is a sysadmin question in a “professional” environment. (Well, not quite professional IMO as I’d consider SAS over SATA, but you use what you have at the workplace.) – mirabilos May 8 '15 at 13:53
  • The Adaptec answer you linked only applies to “wide” connectors, where there is only one cable, not two separate. I found that too, but it does not help here. Note how it says “the power and data pins connect” (emphasis mine), meaning at the same time, which is not an answer to this question. – mirabilos May 8 '15 at 15:29
  • The correct answer would be the one from your motherboard. What brand/model it's ? You can check sata-io.org too, it's the official site for sata rfc, but the doc talk about staggered pin too (like the adaptec answer), not cable order – yagmoth555 May 8 '15 at 15:40
0

I've always thought it makes sense to connect the data cable first. When the drive is off, there will be no noise when you connect the data cable. (That's my 2 cents).

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