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Is it possible to connect two 2.5" HDD enclosures to a Raspberry Pi B+ with only one USB cable, or will I have to use both cables?

I am asking this because I do not actually own a Raspberry Pi and would like to know if I can do it before I buy.

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    I am unclear exactly what you mean. How can you connect to enclosures using only one cable? Presumably each enclosure has its own cable, without which it will not work. Do you mean one cable each? Or do the enclosures come with a two headed USB cable because of power demands? Either way hard drives should be connected through a powered hub to avoid power issues as described in @frenchrh's answer. – Steve Robillard Nov 7 '14 at 6:38
  • @SteveRobillard What I mean is that each HDD has two connectors on the PC side and one on the other side. From what I read only some devices need both connected, as one is just to ensure enough power is getting to the HDD. So I was wondering if the RPi needs both plugged in or not. – flyingfisch Nov 7 '14 at 11:53
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    No one can definitively answer that without you particular hardware. But using a powered hub is recommended . Also, this will eliminate the need to worry about how many ports the enclosures will use. While some drives will work with a single connector, ask yourself this why would the manufacturer go to the extra effort and cost to add the dual connectors if they didn't think they were necessary. – Steve Robillard Nov 7 '14 at 12:17
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You can use 1 powered USB hub, that has its own ac adapter, and then connect 2 usb drives. Leaving the Pi's 2nd USB port available.

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    The OP will be using a B+ which actually has 4 USB ports. – joan Nov 6 '14 at 22:31
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Even with the second cable, the Pi is unlikely to be able to power a single hard drive, let alone two.

The hard drives have a second cable because they can't get enough current via a single 500mA cable, while the B+ pi can by default support up to 600mA (depending on the input power supply) spread across all 4 USB ports. Under worst-case scenario, your hard drives would want 2000mA of current, which is well above that. Now this can be improved, assuming a VERY good input power supply for the Pi, you can modify the config.txt file and bump it up to 1200mA, which is still well short of the 2000mA, but enough to power ONE hard drive, provided that it uses BOTH cables.

The fact that the hard drives require a separate cable means that they require a separate cable, regardless of what system (pi or otherwise) you plug them into, and the Pi isn't going to be able to overcome this restriction.

  • what if i connect the cable one end of each cable to a USB wall charger? – flyingfisch Nov 7 '14 at 20:01
  • @flyingfisch that would be a bad idea, as you would likely fry something - basically you are combining two 5v sources, the Pi and the wall charger, and if one is ever so higher than the other, there will be a current from one to the other - power supplies generally don't tolerate backfeed well, and will either break or have some sort of safety that turns themselves off. – user2813274 Nov 7 '14 at 20:29
  • ah ok, so I should probably find a self-powered external HDD? – flyingfisch Nov 7 '14 at 21:02
  • @flyingfisch that or just get a decent (2+ amps) powered USB hub and connect the hard drives to that (both cables) - if it's large/powerful enough (3+ amps), it can also power the Pi as well – user2813274 Nov 7 '14 at 23:33

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