Go for Seagate GoFlex Ultraportable external HDDs. I have one 500GB drive that came with a USB2 adapter/cable. I also have one 1.5TB GoFlex ultraportable drive that came with a USB3 cable/adapter. The cable/adapter is interchangeable so that you can use the drives with USB2 or USB3 ports. The three screenshots that give the transfer speeds is below.
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The fourth one which is "As a USB2 device into USB2 port", the transfer speed is the same as in the third screenshot. ( can't upload this since maximum allowed is three

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You can see all and the conclusions in my post at Transend USB 3.0 PCI Card - Page 2 - Windows 7 Forums