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I have an old XP laptop with 2 big powered external usb hard drives (1 TB) each. Either one works correctly by itself, but the second one I plug in does not get recognized. I wanted to copy the data from one to the other, but it does not work and I don't understand why.

Any insight appreciated.

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Does the external HDD not recognized by a port or all ports have the problem? When you started to get the problem and what error you are getting will be helpful to help you. – avirk May 16 '12 at 1:06
What is the model of each external hard drive? Are they powered by the USB port alone, or must you also plug them into a power socket? – iglvzx May 16 '12 at 2:15
they must also be plugged into power sockets – glass May 16 '12 at 8:43
When you plug the drives in do they grab the same drive letter - in other words - when you plug drive 1 in does it grab E: drive and then when you remove it and plug drive 2 in does it grab E: as well? – Blackbeagle May 16 '12 at 21:08
yes, one is I and one is H , regardless of usb port – glass May 17 '12 at 11:40

It is probable that the power drawn by running both USB drives together exceeds that available from your USB controller. If you can run each drive by itself, but not both at the same time, then I would try getting a powered USB hub. Older systems may not deliver the amperage necessary - that's why many USB powered drives have split USB cables - they try to take power and data off one USB slot and power-only off another slot.

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they both must be plugged into power sockets as well, so i don;t think this could be the problem – glass May 16 '12 at 8:44

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