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I own an external HDD and it is shipped with a USB 3 Y-Cable pictured below.

I know that the auxiliary branch is to provide sufficient power for the device to work properly. My question is what would happen if only one of the connectors is connected? Would it damage the device or would it make perform slower? Or something else?

USB 3 Y-Cable

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2 Answers 2

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USB ports have a specific voltage and current ratings, see this entry on Wikipedia for the specifics. What this means, is that the maximum power that can delivered by a single USB port is capped.

One way to overcome this power cap is to use an external power adapters and many vendors opt for this solution, esp. for the larger capacity drives, however, this is solution is not very portable (you need to carry the power brick wherever you take the drive).

An alternative is to draw power from multiple USB ports, this effectively doubles your power-cap (atleast on paper). Some disks may even use the additional port to improve transfer rates, but I'm yet to come across any such disks.

Now as to what would happen if you use only a single connector is anybody's guess... the disk could operate just fine for some time and then tank, it might not spin up/be identified at all or it may malfunction in other ways. Generally, it would be a bad idea to use just one connector when two are provided.

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If you buy an external case for a bare drive, they ship with a Y connector. The instructions often say that low-capacity 2.5" drives can get by with one connector, larger drives need two. There is a grey area between work and not work, where a drive will spin up if the current is inadequate but the performance may not be reliable. If a drive ships with a Y, that's a good indication that both are needed. If you are short on USB sockets, you can plug the power connector into a USB 2.0. They sell Y's where the power connector is a pass-thru so you can use the socket for a low-power device. –  fixer1234 Oct 18 at 3:23

Dependant on the power output of your USB port, the device may or may not spin up. I have several older USB-2 external drives with the same kind of cable arrangement and some need both connectors plugged in, others will work with just the primary.

I think in summary, you should be OK to plug it in and try it. If it spins up, it'll work fine, if not, then you're going to need the other connector.

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So you say no harm can be done by trying only one connector? –  Hamed Momeni Dec 22 '12 at 7:56
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I'd be amazed if that were the case, although obviously I'm not going to guarantee it. I've certainly never had any issues and I have some real old kit knocking about. The worst I've seen is the device just doesn't power up. –  FreudianSlip Dec 22 '12 at 8:00
    
Ok Thanks. I'll wait for others to weigh in to have a more concrete answer. –  Hamed Momeni Dec 22 '12 at 8:10
    
My guess is that it would work if you used a USB 3.0 port, due to the increased current it can provide. I don't know for sure, though. –  user55325 Dec 22 '12 at 8:23

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