USB power and external harddrive power draw

Posted on 2007-07-22
Last Modified: 2013-11-14
So I've been doing some research and I've run into a problem that requires clarifying (mostly because I don't understand the finer points of electrical components). I have a WD Passport external harddrive. After reading up on it, the failure of the harddrive (and other Harddrives powered soley by USB) seems to occur because some USB ports do not provide enough power for what is required (up to 650 mA). However, this seems to imply that there is a USB configuration that *would* satisfy this power requirement. My understanding was that USB had a ceiling of 500 mA. Logic would suggest that a single usb cord (as opposed to the dual slot power boosters) can never satisfy the upper limits of such a device's power draw. My question is simply: is this a correct assumption? Can anyone offer clarification on the issue? Should I buy a dual usb power boosting cable for my harddrive? And how would a dual USB cord power the device over 650 mA if that power is coming from the same USB hub pins on the motherboard?
Question by:lake59
    LVL 91

    Assisted Solution

    >>  some USB ports do not provide enough power for what is required   <<  you are right about that ! usb power is limited to 500 mA - so some use 2 usb connectors to draw more.
    i always suggest external devices with their own power connector, just to be safe.
    Once the usb bus is burned, you may need a new mobo, if you are unlucky
    LVL 87

    Expert Comment

    Many such devices also have a separate cable which is connected to another device, like the PS2 port of a PC from which it also gets power. Another option is to use a powered USB Hub.
    LVL 2

    Author Comment

    Except I'm wondering how plugging in two USB cables is supposed to power the device sufficiently if the hub is still limited to 500mA - or is that a false assumption and rather, the power limit is for the individual ports?
    LVL 87

    Accepted Solution

    The hub may have higher power. PC's just only have to deliver the 500mA, and they are often set to deliver only very close to that. For the Powered Hub it would depend on the Power supply used, it should deliver as much that the supply can deliver.

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