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I am getting my RasperryPi Rev2 soon, and I plan to use it as a media centre. I have a WesternDigital Elements 1TB USB3 2.5" USB powered external harddrive. I know that a normal USB port outputs 500mA, and the Pi recommends 700mA in.

I was wondering if I bought a 1.2A PSU, will that be enough to power the external through the RPi's USB ports?

I was looking at this power supply.

Micro USB power supply adapter,Euro,1.2A

Thank you for your help.

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  • Is four cables really too many?
    – Ash Burlaczenko
    Dec 31, 2012 at 20:50

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The 500mA is for USB2 (max of 5 loads of 100mA), this drive is USB3 (max of 6 loads of 150mA = 900mA).

External drives often have a (Y or chained) cable that connects to 2 USB ports to get the power needed. Check the specifications of that drive very well before buying anything.

A second reason why that adapter is too tight fit is the surge current (inrush current), a lot of power (relative) is needed to get the disk spinning inside, his is only for a short moment, but in this short moment the current is high, the voltage level might collapse to such a low level that the RPi starts to act weird, or just reboots.

The most safe option is to power the RPi with a separate supply and connect the drive through a powered USB Hub. This ensures that the RPi will not have to deal with any effects from power fluctuations from external devices.

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  • Thank you for your quick reply! I'm trying to avoid extra cabling and components. I have looked for the current that the external requires - can't find it on the packaging, package insert or the drive itself. Could I avoid the powered USB hub and get a more appropriate PSU, or should I get the powered hub?
    – KCranky
    Dec 31, 2012 at 12:13
  • The 1.5A is a very tight fit, I would go for something a little higher. But how high depends on the specs of the drive. Having a 2.0 board you could also go for another option, back feed the RPi from a USB Hub, you still need only one PSU (of max 2.5A) connected to the Hub and it will power both drive and RPi. Maybe the safest and most elegant solution, also if you suddenly want to connect an extra USB device (keyboard or mouse for some testing or in the case the RPi seems to hang) you can connect those without a problem.
    – ikku
    Dec 31, 2012 at 14:19
  • So plug in the powered USB port to power. Plug in the Pi to the powered port, and the port to the Pi?
    – KCranky
    Jan 1, 2013 at 9:34
  • I think the drives are supposed to be backward compatible with USB2.0. In That case they should not need more than 500mA
    – John La Rooy
    Mar 1, 2013 at 13:26
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The USB ports on rev 2.0 no longer have the silly poly fuses, but the power to the ports still goes via the main polyfuse.

Increasing the powersupply isn't a great idea, because of this polyfuse and also because it exceeds the specs of the USB socket the PI uses for power.

You should either use a powered hub (yuck) or splice the power cable to the drive (also yuck as this defeats the original purpose of the polyfuse)

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