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I think the point is that in the USB handshake, it actually tells the computer how much power it's taking so that the computer can allocate power properly. If it just sucks power without telling the computer, it might oversubscribe the power.



Each port must supply 500mA at +5VDC, so there's no oversubscription.

And they have to supply that power before anything is plugged in, since the device needs to power up to talk to the host to tell how much power it really needs.

So one port will be reporting a full draw of 500mA presumably, and the other port will just behave as if nothing is connected to it.. drawing up to 500mA regardless.

This is mostly just for spin-up current. Once spinning, 500mA is about what most notebook drives need to keep spinning (2.5W or so).

Cheers