Yes when you say Old system it may have been in the days of plug and pray!!    The latest external cases are a doddle to put together all you do is connect the HD  to the cable Your will be covered for warranty in the same way with the products you buy this way!   If one of the units is going to be for older OS then you may find the same recognition problems,,   I had the same problems until I installed service pack 2 which resolved  problems with windows XP.. Anyway the cased versions are not that much more expensive but may not give you the choice of a quality HD that you will have if you select your own devices to build, some Drives are far more reliable and speeds can vary a lot..

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt