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I have looked at the Western Digital website for my product (WDBACX0010BBK) and cannot find how much power the device requires.

The device is not recognized when I plug it into my keyboard's USB hub. The keyboard manufacturer told me that each port supplies up to 100mA.

When I plug the drive into my desktop, it works just fine.

Can anyone link me to a resource that specifies the power requirements for my external hard drive?

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A powered USB port will supply up to 500 mA (anything on a direct bus in a PC or any hub that has power supplied via wall outlet), a non-powered one (like the one in your keyboard) will supply only 100 mA. Chances are that it uses somewhere between 100 and 500 mA. If it were more than 500 mA it would require separate power. – MaQleod Sep 8 '11 at 15:21
up vote 2 down vote accepted

It does specify the interface as USB 2.0 which is 500mA max, the hub that is in the keyboard evidently can only support 100mA, which is common for keyboard hubs, and is most likely a USB 1.1 hub also. USB hard drives that do not have an external power adapter (common for 2.5" sized external hard drives) require more than 100mA due to the fact the hard drive has spinning platters and a motor to drive them.

I have a Dell keyboard with 2 usb ports, I connected my flash drive to one of them and got this message which verifies it is indeed a USB 1.1 hub.

. enter image description here

Then I went into device manager and found the keyboard hub then I went to the power tab for that hub, which shows the max mA per port.

enter image description here .

Now here is where I am baffled, I connected my 2.5" usb hard drive to my keyboard and it did not work, no message in windows, nothing, then I connected it to the USB port on the PC and checked in device manager to see what it required for power, it said 100mA, maybe it is some other limitation of the keyboard USB port or device manager is wrong?


enter image description here

Next I used a USB Y cable to get power from 2 usb ports, connected the Y cable to both keyboard ports, then connected my 2.5" drive, still nothing, the drive was cycling power spinning the drive up and down, then I took one of the Y cable connectors and connected it to a usb port on the PC, plugged in the hard drive and now it works as expected, so it must be some sort of power issue with the keyboard hub.


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Thank you for teaching me that there was a power tab on the properties of my usb hub; I didn't know that! Indeed, my USB Mass Storage Device lists 500mA as the amount of power required! – John Ruiz Sep 8 '11 at 23:32

Well, there is a power specification provided by all USB devices. But if you know the USB protocol well, you will find that the power requirement provided is not calculated by the computer (host). The power requirement must be transmitted by the device to the host. So if a device sends that it needs 100mA, it may still actually need 300mA.

Since the data can be unreliable in many cases, it is better to see what the manufacturer ratings say for the device. Otherwise simply assume 500mA for relatively heavy electronics like HHD or vacuum cleaners, whatever.

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It indicates that the "My Passport" 2.5" drive series requires a USB 3.0 port with a minimum of 650 mA available.

I don't know how different other drives in the Passport series are but 100 mA is insufficient for most drives in my experience. I think USB 3.0 supports up to 900 mA per port.

I wouldn't trust the device manager properties tab as I get 500 mA quoted on a port that I know supports 900 mA. Go figure.

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