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How capable are USB hubs?

I have an AirPort Extreme router with a printer attached (it's not powered by USB). I want to extend this and add two hard drives (one for Time Machine and the other for EyeTV recordings).

Can a 4-port USB hub (I'm considering this one) achieve USB 2.0 speeds and power the hard drives? What difference would a self-powered vs externally-powered hub produce?

  • As well as the hub do pay attention to the quality and suitability of connecting cables. – mas Jul 17 '09 at 15:33
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Ive run 3 external hds off a Belkin wall-powered hub and gotten the same speed as a direct connection, however my hard drives were all wall-powered as well. As a rule of thumb, it's definitely better to get powered USB hubs for any hard disk usage or heavy data access (bigger thumb drives, etc), while self/computer powered hubs are usually for mouse/keyboard and generally device inputs. There is no downside as far as I am aware of using a wall-powered hub, so if you have the extra cash, its always a good investment to go with external power. I personally use an old model of Belkin's hubs, and its served me well for almost 3 years now.

tl;dr: Drawing usb power is bad for lots of data transfer, grab an externally/wall powered hub and you should be fine. Try and get the hard drives themselves powered too -- the less stress on the hub, the better.

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    Does this still make a difference if the HDs themselves are externally powered? I always assumed you could use an unpowered hub if all the devices you were using were self-powered. – stillinbeta Apr 23 '10 at 14:59
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A self-powered USB Hub typically reinforces and repeats the signal so that you may have longer cables. It's definitely to be prefered when running heavy USB appliances such as harddrives.

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    USB cable length is limited by signal roundtrip time, not signal strength, so you can't get far beyond the 5m limit, no matter what cable/hub you use. Stability problems are likely caused by devices drawing power beyond specification. – Martin May 3 '11 at 17:46
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Each device reports to the hub its power requirements, and won't get initialized if it asks for more power than is available. Also, each standard USB 2.0 port can be asked to provide up to 500mA. If you have a self-powered hub, then the hub plus all devices attached must draw less than 500mA. A powered hub would be able to provide additional current to each connected device. Recommended: a powered hub, especially if you want to connect a self-powered hard drive.

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How capable are USB hubs? Their capabilities vary with manufacturers. A good (and USB Certified) USB hub would be self-powered (getting power from a wall adapter, not from the host port), will have individual port power protection/control. And will have an official USB-IF Certification logo. These hubs still sell for $40-$50.

A cheap (bus powered) USB hub would take the power to supply all connected peripherals from upstream (usually non-detachable) cable. Technically speaking, these "minimized" hubs (most of them) a cheats, because they report in their descriptors that they are "self-powered", thus lying the system. A bus-powered hub can/should supply no more than 100mA per port, and the system will deny any USB device that describes its power consumption above this limit (you will have a pop-up error message). So to be useful and avoid system power policy, these el-cheapo hubs must lie to the system. In addition, the port where this hub is connected to must have a lot of current sourcing capability to drive all four suggested devices (if they are NOT self-powered from wall-type adapters).

Bus-powered hubs have one big disadvantage with regard to quality of power delivery - power to any new attached device will go through the same piggy-tail upstream cable. As result, the initial voltage "droop" (on connect) will affect all other already connected devices, so they might disconnect. But if you are not planning to use hot-plug devices while the rest is operating, this might be of lesser concern.

In short, the Targus hub will give you full USB2.0 functionality on all four devices if all hard drives are "self-powered" with wall adapters. If not, you might experience functional instabilities. Or might not, all will depend on quality of your host port.

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