Normally you stick the HDD in a USB enclosure (around 20 Dollars).
.. 3 1/2 inch HDDs (desktop models) require a Powered source as they require 12V
.. 2 1/2 in HDDs (Laptop HDD) only require +5 V and in many cases can be powered from a USB port using a sata->usb cable. HOWEVER I recommend using the enclosure that is powered from a AC adaptor or a powered usb Hub. Reason is that USB 2 ONLY provides upto 1/2 amp which may NOT be enough power.