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I have a freecom external USB HDD, which wont run from a powered hub, so I imagined it was something to do with the powered hub not supplying enough power. However when I connect it directly to a Linux machine, and issue the command:

lsusb -v|egrep "^Bus|MaxPower"

I get the following result:

Bus 002 Device 003: ID 07ab:fc88 Freecom Technologies 
    MaxPower                2mA

(Windows Reports the same thing in fact)

Perhaps that's the power drawn by an internal hub? How can I tell how much power is really being used?

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Why don't try to determine this another way. Determine the power requirements for the device itself. You can think determine if you need a powered hub that is closer to 3MA. You also didn't indicate the watts. –  Ramhound Sep 6 '13 at 12:07
@Ramhound thanks for your comments. The documentation of the device does't appear to say how much power it needs. Watts=Volts*Amps, and I assuming its 5V. 2mA is obviously way to little - the drive must consume more than that. A USB 2.0 port should be able to provide 500ma. I have a 4 port powered hub with a 2000mA (i.e 2A) power supply. However I am aware some devices require more than 500mA and I suspect my drive is one such device. –  nwaltham Sep 6 '13 at 12:50
Provide the volts on the power supply for the hub. The best you can do is get a hub that has a 3mA power supply and test it. There should be a label on the device itself that has the power requirements, unless this isn't, a product normally sold in NA/EU –  Ramhound Sep 6 '13 at 12:57
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It looks like the device is lying to you about how much current it draws. 2mA seems several orders of magnitude too low for a bus-powered external hard drive. The MaxPower value is reported directly from the device and this wouldn't be the first time that a device manufacturer cut some corners and reported an inaccurate value.

Unfortunately there isn't a reliable way to measure the current draw of a USB device with software, which leaves only hardware solutions. If you don't care about measuring the actual current draw, and just want the device to work, I'd recommend trying a USB Y cable to see if 2 USB ports on your hub can provide enough power.

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