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I'm using a 2.5" SATA disk, via an enclosure for eSATA on my laptop. It came with a USB powered adapter, which has two USB ends.

Since I don't have an eSATAp port, I need the power from the USB plugs, but I noticed that it works with either one or both plugged in.

Are there any advantages or disadvantages in using a single USB power or both? I imagine that if it didn't get enough power, it would simply not work.

I have not noticed any performance degradations, and I would really like to utilize the extra USB port.

Also, does this apply to USB powered USB to SATA enclosures/disks?

edit Since this not easy to answer generically, i'll ask specifically for my ST9500420AS drive, connected via either 1x USB 2.0 or 2x USB 2.0 slots, running under eSATA.

edit with bounty Layman terms: If it works with 1 USB cord and with 2 USB cords, does that mean that it will work slower with 1 cord? Or does it mean that since it's on, it's working at full capacity?

benchmark results Thanks to the answers, indeed 1 USB is enough if it manages to work.

Here are my benchmark results:

CrystalDiskMark 3.0.2 x64

1 USB for Power

         Sequential Read :    83.832 MB/s
        Sequential Write :    78.545 MB/s
       Random Read 512KB :    29.415 MB/s
      Random Write 512KB :    35.368 MB/s
  Random Read 4KB (QD=1) :     0.369 MB/s [    90.0 IOPS]
 Random Write 4KB (QD=1) :     0.639 MB/s [   156.0 IOPS]
 Random Read 4KB (QD=32) :     0.714 MB/s [   174.3 IOPS]
Random Write 4KB (QD=32) :     0.651 MB/s [   158.9 IOPS]

Test : 500 MB [E: 93.7% (321.4/343.1 GB)] (x5) Date : 2013/05/20 0:26:04 OS : Windows 7 Professional SP1 [6.1 Build 7601] (x64)

CrystalDiskMark 3.0.2 x64

2 USBs for Power

         Sequential Read :    84.155 MB/s
        Sequential Write :    79.341 MB/s
       Random Read 512KB :    30.546 MB/s
      Random Write 512KB :    28.847 MB/s
  Random Read 4KB (QD=1) :     0.238 MB/s [    58.0 IOPS]
 Random Write 4KB (QD=1) :     0.592 MB/s [   144.6 IOPS]
 Random Read 4KB (QD=32) :     0.672 MB/s [   164.0 IOPS]
Random Write 4KB (QD=32) :     0.635 MB/s [   155.0 IOPS]

Test : 500 MB [E: 93.7% (321.4/343.1 GB)] (x5) Date : 2013/05/20 0:34:06 OS : Windows 7 Professional SP1 [6.1 Build 7601] (x64)

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The disadvantages is the performance issues you experiened. –  Ramhound Apr 30 '13 at 14:58
Does the external drive have two USB inputs? What does the manual recommend? –  Brad Patton Apr 30 '13 at 15:04
Here's the answer you're looking for: superuser.com/a/193969/218300 –  M.Bennett Apr 30 '13 at 15:37
sorry, i meant i have not noticed any performance degradation. I will try and benchmark myself to see. –  ericosg Apr 30 '13 at 16:45
@ericosg - It really depends on the exact drive being used inside the enclosure. A general statement would be incorrect in many cases. A green drive for instance already requires less power, a higher speed 10,000 RPM drive will require more power, the power across USB 2.0 is different then USB 3.0. –  Ramhound May 1 '13 at 11:03

6 Answers 6

up vote 5 down vote accepted

USB2 specifies minimum amounts of available power (600 mA, 4.75V, so 2.85 Watt). Your PC probably supplies more than that. In particular, it almost certainly will supply 5 Volt (3 Watt).

That's sufficient for your particular drive. However, there are plenty of drives which require more power. 4 or 5 Watt is quite common. That also means some drives will require one or two USB connections depending on the voltage of the PC. This can cause hard-to-diagnose errors.

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What I am trying to understand is: If it works with 1 USB cord and it works with 2 USB cords, does that mean that it will work slower with 1 cord? Or does it mean that since it's on, it's working at full capacity? –  ericosg May 16 '13 at 12:48
@ericosg: No difference in speed. The main power draw is when the disk spins up. This may be affected. Once it spins, and you read the first byte, one USB cord will be enough. –  MSalters May 16 '13 at 21:48
So you support that if it works with 1 cable, there is no performance gain to add a second? –  ericosg May 18 '13 at 4:37
@ericosg: exactly. The worst it can happen is that the spin-up will fail (or the disk will appear to spin down and up at boot) if the current is not enough. Once it is up to rotation speed, the data speed will stay constant. There is no "low power data transmit". –  lserni May 19 '13 at 19:16

The two USB's are solely for power, not for data transfer. So if 1 USB is capable of powering it (as you have said is the case) then there is no benefit in plugging in the second one.

Situations that would require the second one include:

  • You are using hardware that the USB ports do not provide as much power as they should (e.g. a battery powered USB port).

  • You are using a larger (physically) and/or faster hard drive that requires more power to operate than the USB spec can accommodate.

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Sounds like what I had in mind. Can you back this up with any documentation or actual tests? Are you 100% sure? –  ericosg May 18 '13 at 4:36
Sure, can you give a link to your cable so I can give you specifics. –  Dudemcman May 18 '13 at 8:54
I'm connecting a ST9500420AS drive with a very standard eSATA enclosure on a laptop with eSATA and USB 2.0 ports. –  ericosg May 19 '13 at 13:22

Hard drives do not use a consistent amount of power. In particular they use as much as twice as much as their "idle" power when spinning up the disk (from power off or sleeping) and more power doing long seeks than doing short ones. So it is completely possible to have a drive that mostly works but has sporadic failures when it is underpowered.

However, your particular drive uses less than 2 watts, which is well within the specification for the amount of power a single USB peripheral can draw from the USB port. So you are safe using just one USB connection.

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I agree with your statement to a certain extent that the hard drive would either work or it wouldn't. I've run into situations where external drives weren't getting enough power and they would make repetitive noises like they were trying to get up to speed but couldn't. My best guess on the two USB connectors is that one is meant for data and the other is used to supply additional power if your computer is not providing enough. Do the connectors have different colors or symbols that differentiate their purpose? It would be helpful to know what enclosure you are using with the drive. Are you using the eSATA connection? The eSATA connection should provide you with better performance and I would hope that the enclosure electronics would default to eSATA if both USB and eSATA connections are present. Either way, you should not see a performance change by hooking up the second USB cable.

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Yes i'm using eSata (mentioned in question too :P). Thanks for your answer but rather inconclusive. –  ericosg May 18 '13 at 4:35

As stated above (but not quite true), one plug is for (extra) power, the other for data. Manufactureres do not know what drives will be placed in their case so often will include the ability to plug into a second port for more power. Some drives need it some don't. Can you afford the chance of having write errors? If not and you want to use your second port for other things you might consider getting a (powered) USB hub. That way you can even plug in the coffee pot. :)

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it sounds like you are implying that if i use 1 port when i needed 2, instead of the device simply not working (my current assumption), i would end up with write errors. is that a fact? tia –  ericosg Apr 7 at 20:35

No one USB end will be fine for powering the drive and transfering data too.

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