You are copying over USB? something is wrong there... copying general files should go around around 10mb a sec min... on a average hard disk... would expect 1Tb disks to do at a avg 20MB a sec... or more..
if your copying hundreds of thousands of tiny files you can really average speeds at times...

Might be useful to know how you have them hooked up, perhaps the external hard disk case if faulty your using? try a different usb port, on the back of the machine perhaps.

Oh and your just copying using windows explorer?