Check the hard drive out tomorrow, on your son’s desktop, and if this is OK then return the enclosure. Take the hard drive along with you and get them to put it into a new enclosure and plug this into a PC to demonstrate that it works before leaving the shop!
I use a USB HDD in an external drive on my laptop. One day I plugged it into my desktop and like you it wasn't detected or seen by the Computer.
However, after much head scratching I assigned it a drive letter using diskmgmt.msc (which could "see" it) [Right Mouse Click] and my problem was resolved.
It immediately became detectable and visible to XP.
sorry took longer as expected to come back - business!
anyway solved out the problem, after a chain of silly things. first jumper was set wrong, usb cable was damaged and at last wrong usb plug on the vaio. yet, operates fine.
