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I'm on the verge of buying myself a new bus-powered USB 3.0 2.5-inch external hard drive. However, my computer only has USB 2.0 ports on it. I understand that the USB 3.0 HDD will simply run slower on USB 2.0 until I upgrade my machine, but my question is this:

Can I be sure that my USB 2.0 port can provide enough power for this bus-powered USB 3.0 device?

I read that the maximum power provided by USB 3.0 is 900 mA, which is almost twice the maximum power provided by USB 2.0: 500 mA. This made me think that even though USB 3.0 is backward compatible with USB 2.0, a USB 3.0 powered device may not work on a USB 2.0 port. If it depends on the device, what would indicate USB 2.0 power compatibility when choosing which external hard drive to buy?

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  • No, I will not provide you with its 3.0 max data transfer speed. – Ishan Shah Dec 30 '20 at 4:32
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    Should work on most decent machines. Not guaranteed though. Check the harddrive specs for the minimum power required and your machine specs for how much power is available through the USB2 ports. Many cheaper laptops and even some desktops won't supply enough power, which is why they even access USB2 drives slower than better machines (and also won't charge your smart phone 😜) Generally, if your USB2 port will charge your smartphone on a plain USB cable, it will also run a USB3 drive, if rather slow – Gwyn Jan 6 at 23:00
  • Provided you used a relatively new smartphone to test ... – Gwyn Jan 6 at 23:01
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I have an USB 3.0 2.5'' Drive (without external power source) from verbatim and when I plug it into my old laptop which only has USB 2.0 it is incredibly slow (much slower than my USB 2.0 2.5'' hard drives of comparable size). My guess is that this has something to do with the power, but I haven't ruled out that it might be some other issue, as I'm not sure how to check that. Needless to say, it runs just fine and much faster than my USB 2.0 drive on a USB 3.0 port.

So, my (statistically not significant) experimental result is: It works, but it might be slow, and I'm not sure if it is good for the hard drive to run on too low power.

And about how to find out: I tried to find out about that by looking at the technical details and comparing them, but there was nothing in there that seemed helpful. If I remember correctly, there were other USB drives that explicitly stated that they work on USB 2.0 ports, and mine didn't, so maybe that is something to look out for, although it also might be a case of asbestos free cereal.

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My personal experience with a 2.5-inch Toshiba Cavino 1TB drive and it works fine in a USB 2 slot. It only has a single USB cable.

The only difference is when it is plugged into a USB 2 port, the activity light is white while in USB 3 port the activity light is blue.

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I found out the hard way that not all USB 2.0 ports are equal. I bought a USB DVD drive some time ago and discovered that not all computers will provide enough power for it to run. As I understand it the USB 2.0 spec requires that a USB-A port on a PC or powered hub supply at least 4.5 watts, or 900 mA. A USB 2.0 device is allowed to draw up to 7 watts, or 1.5 amps, and not all USB ports will provide this much power.

There's the USB-BC (battery charger) spec that allows up to 12 watts, or 2.4 amps, out of a USB-A port. With people expecting more power from their USB-A ports, such as for charging up their cell phones, I'm seeing more manufacturers put more power to their USB ports. I don't know what year this switch was made and this may not be documented in the consumer facing spec sheets on a computer. I can test this only by plugging in my 1.5 amp DVD drive and seeing if the computer will supply enough power. There's certainly better test equipment available, I just don't see the need for me to invest in such.

As you have discovered USB 3.x requires that USB-A ports supply more power at a minimum. The maximum power a USB 3.x device can take, again as far as I can tell, is still 1.5 amps. And again with people expecting more power from their USB-A ports now than in the past it's my experience that most USB-A ports today, USB 2.0 or USB 3.x, will supply more than 7.5 watts. I'll see claims of up to 3 amps available from a USB-A port but I'm not sure that more than 2.4 amps is allowed by the USB spec, and that appears true for any version of USB.

The power drawn by any device should be documented somewhere. That DVD drive I mentioned has on the bottom that it requires 5 V @ 1.5 A. I happen to have a 2.5 inch hard disk drive in reach and it has on it that it takes 5 V @ 700 mA. The external case that the drive is in (which is helpfully transparent which is how I was able to read the label so quickly) supports USB 2.0, USB 3.0, and FireWire. While it has a power input I haven't needed it yet (and if I did want to use it I'd have to find a power supply as one was not included in the box).

You may be able to find out how much power the USB 2.0 ports on your computer supplies by looking in the spec sheet, but that might not be documented anywhere for the consumer to see. Perhaps you can experiment like I did with a device, or devices, of known power draw. Perhaps you can run some software to see how much power a device takes. As an example I can run System Information on my MacBook and see that my iPhone is allowed to consume up to 2.4 amps. I plugged in in with a USB-A adapter to the USB-C port so it could not take more power than USB-A would allow.

I know I didn't give much more than you can't know until you try. I do know that some older USB 2.0 USB-A ports won't allow more than 900 mA. Most anything made since people started plugging in cell phones into their computers will supply at least 1.5 amps. The most you can expect is 2.4 amps as that is all the USB spec allows, unless there's been a change I am not aware of. Hopefully this is documented clearly for you somewhere, if not then you are going to have to experiment.

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You can use a dual head USB cable (has two USB plugs to split the load between the USB ports).This was common with usb2.0 cables back in the day .You might get away with one cable head, if the HDD is a 5400rpm drive and you use the front USB on your pc , because in my experience/observation the cable that supplies both ports on the front(usb2.0) has its power trace on the front USB board joined together.

Edit: This is incorrect , if we check the usb spec from https://compliance.usb.org/index.asp?UpdateFile=Policies#72 it mentions

Use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral. If a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered.

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  • Please don't use a USB Y-cable. This is bad form, not supported by the spec which can mean unreliable and unpredictable behavior, and potentially damaged hardware. The proper means to handle this is with an external power supply. With USB-C ports able to supply up to 100 watts there should be no need for a hack like a USB Y-cable. – MacGuffin Dec 30 '20 at 3:01
  • I guess i was wrong , updated my answer – binaryOps20 Jan 6 at 22:15

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