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up vote 15 down vote favorite

I know USB 3.0 is almost entirely backward-compatible, and I know that it introduces a new speed that USB 2.0 devices aren't capable of, but is there any advantage to having a USB 2.0 device in a USB 3.0 port?

Though I'm interested in if it would provide any benefit for any device, I was specifically thinking of a USB Hub that I plug my Bluetooth receiver and flash-drives into.

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3  
No. The USB device that you are using cannot go faster. – soandos Jun 5 '11 at 16:06

5 Answers

up vote 19 down vote accepted

Since your USB is optimized for USB 2.0, using a 3.0 will see no improvement because it simply cannot operate at 3.0 speeds.

USB 2.0 has a maximum speed of 60 MB/s USB 3.0 has a maximum speed of 625MB/sec

From Wikipedia's Article on the Universal Serial Bus:

Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet.[62] This is 70% of the total bandwidth available.

Based on this, you can see that USB 2.0 devices just are not capable of the speeds 3.0 has to offer.

TL;DR version: You will see no benefits

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That quote is unrelated to the question; it's about USB-hard-drives specifically, and is saying that even USB 3.0 hard-drives won't benefit from plugging into a USB 3.0 port, because they don't saturate the available USB 2.0 bandwidth as it is. There could very well be devices (such as flash drives) which could otherwise operate at USB 3.0 speeds, but can't simply because they are not USB 3.0-compatible. – BlueRaja Jun 5 '11 at 22:09
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@BlueRaja it was meant to be taken as an example. – Simon Sheehan Jun 5 '11 at 22:41
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+1 for the TL;DR version. Though I had already read the whole answer :/ – Gani Simsek Jun 6 '11 at 9:38
@BlueRaja: It may be saying that USB hard-drives don't saturate USB 2.0, but that would be wrong. USB doesn't allow any single device to use all the bandwidth, and the transfer rates seen by USB disks are very close to the theoretical maximum for any USB 2.0 device (once protocol and overhead are considered). – Ben Voigt Jul 1 '12 at 0:10
@BlueRaja Actually, the mechanical hard drive is typically capable of saturating USB 2.0. 70% sounds about right, accounting for overhead. As a general rule of computing, networking, etc., you will never hit the theoretical limit of any interface. In any case, if the HDD interface is USB 2.0, a USB 3.0 port will not raise the theoretical maximum. – Bob May 1 at 5:57
up vote 1 down vote

Actually, yes, it will be faster by a small margin. You will only see gain if the device in question can dish out a higher bandwidth over another interface like ExpressCard or PCIe. for instance a modern 7200 hard drive in a external enclosure could more than saturate the USB 2.0 port. If the enclosure is a USB 2.0 device, it will be operate with more of its bandwidth when plugged into a USB 3.0 hub, but not nearly as much as if it was a USB 3.0 to USB 3.0 device to hub link (with a USB 3.0 cable).

At least on my laptop, USB 2.0 external 500 GB on USB 2.0 gives me about 19–23 MB/s and up to 25–32 MB/s when connected to a USB 3.0 express card. So I am getting both a higher minimum speed and ceiling when the same USB 2.0 device is on the USB 3.0 hub. I think the controller is probably more efficient on USB 3.0. When I plugin a USB 3.0 thumb drive on the same ExpressCard USB 3.0 hub though, I get up to 122 MB/s.

So short answer; yes, a small increase, but not nearly as fast as native USB 3.0 links.

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up vote 4 down vote

At least on my computer, the 3.0 port is faster than the 2.0 port. However, that is because it is on a different controller, which is faster than the one built into the chipset.

Basically, if the usb 3.0 is from the same controller as the usb 2.0 port, it's going to be the same. Otherwise, YMMV.

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This is the only right answer. USB3 hardware tends to be more sophisticated than USB2 (faster microcontrollers, more cache memory, faster PCIe interface) and this might translate to benefits for high-speed devices. – Ben Voigt Oct 9 '11 at 19:21
up vote 10 down vote

One advantage could be that USB 3.0 can supply more power than USB 2.0.

I have some doubt whether an USB 2.0 device could use that power, as it would be designed for a USB 2.0 port. On the other hand many USB 2.0 devices exceed the specified power and get away with it (mostly external disks when they start up).

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Some USB 2 devices have problems when connected via a hub. Would be interesting to know if they have fewer problems if the hub is plugged into a USB 3 socket with its higher power rating. – pelms Jun 5 '11 at 22:53
I'm gonna open this question back up and hope someone has any definite answer about this. – Nich Del Jun 5 '11 at 23:49
This is interesting because technically (according to specs) USB 2.0 can only draw one unit (100 mA) without negotiation, and up to five units (500 mA) upon request. USB 3.0 can draw one unit (150 mA) without negotiation and up to six units (900 mA) upon request. Thing is, many USB 2.0 devices don't negotiate properly. And many computer motherboards just supply power without caring about negotiation. So the correct answer would be that it's more likely that a USB 3.0 port could supply higher amounts of power, but a port fully following standards does not have to supply that amount. – Bob May 1 at 5:56
up vote 7 down vote

None, other than it will work. You gain no performance unless using USB 3.0 devices.

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