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#11
Even the fastest HDD will not saturate a USB2 interface, so you will not get any benefits from connecting it with a USB3 cable. In addition, although the USB3 sockets will accept USB2 cables as well as USB3 ones, the same is not true of the reverse, so it is extremely unlikely that a USB3 cable will fit your external enclosure anyway.
Assuming that you do have an enclosure that accepts a USB3 cable, you still won't get an increase in speed when using a HDD due to it being an electro-mechanical device. For that, you require an SSD, and even those will not saturate a USB2 interface (although they are getting closer).